Twins as a natural experiment to study the causes of mild language delay: I: Design; twin-singleton differences in language, and obstetric risks

The mother during pregnancy and the puerperium: Detailed data abstracted from the clinical obstetric records of ALSPAC pregnancies

Background

When the Avon Longitudinal Study of Parents and Children (ALSPAC) was planned, it was assumed that the clinical obstetric data would be easily accessible from the newly developed National Health Service computerised 'STORK' system. Pilot studies, however, showed that, although fairly accurate in regard to aspects of labour and delivery, it was, at the time (1990-2), inadequate for identifying the full antenatal and postnatal details of clinical complications and treatments of the women in the Study.

Methods

A scheme was therefore developed to train research staff to find and abstract relevant details from clinical records onto proformas designed for the purpose. Extracting such data proved very time consuming (up to six hours for complicated pregnancies) and consequently expensive. Funding for the enterprise was obtained piecemeal using specific focussed grants to extract data for subsamples of the Study, including a random sample to serve as controls.

Results

To date, detailed records have been completed for 8369 pregnancies, and a further 5336 (13,705 in total) have complete details on specific prenatal areas, including serial measures of maternal blood pressure, proteinuria and weight. In this Data Note we describe the information abstracted from the obstetric medical records concerning the mother during pregnancy, labour, delivery and the first two weeks of the puerperium. Information abstracted relating to the fetus (including fetal monitoring, presentation, method of delivery) and neonate (signs of asphyxia, resuscitation, treatment and well-being) have been described in a further Data Note.

Conclusions

These data add depth to ALSPAC concerning ways in which the signs and symptoms, procedures and treatments of the mother prenatally, intrapartum and postnatally, may impact on the long-term health and development of both mother and child. They augment the data collected from the mothers' questionnaires (described elsewhere) and the 'STORK' digital hospital data.

Speech processing performance of Hungarian-speaking twins and singletons

Studying speech processing in twins versus their singleton peers provides opportunities to study both genetic and environmental effects on how children acquire these aspects of their speech and - by extension - their phonological systems. Our study focused on speech processing in typically developing Hungarian-speaking twins and their singleton peers between 5 and 9 years of age. Participants included 384 monolingual Hungarian-speaking children (192 twins, and 192 singletons). Data from four tasks - repetition of synthesised monosyllables, nonsense words, well-formed noisy sentences, and well-formed phonologically complex sentences - were analysed. There was a main effect for birth status, and singletons outperformed their twin peers on the majority of the speech processing tasks. Age and task also had effects on the performance of the participants, and there was a three-way task by age by twin versus singleton status indicating that the speech processing performance of twins versus singletons is interdependent with the type of task and age. Our results also indicate that monolingual Hungarian-speaking twins may be at higher risk for developmental speech delays relative to their singleton peers.

How does linguistic context influence word learning?

While there are well-known demonstrations that children can use distributional information to acquire multiple components of language, the underpinnings of these achievements are unclear. In the current paper, we investigate the potential pre-requisites for a distributional learning model that can explain how children learn their first words. We review existing literature and then present the results of a series of computational simulations with Vector Space Models, a type of distributional semantic model used in Computational Linguistics, which we evaluate against vocabulary acquisition data from children. We focus on nouns and verbs, and we find that: (i) a model with flexibility to adjust for the frequency of events provides a better fit to the human data, (ii) the influence of context words is very local, especially for nouns, and (iii) words that share more contexts with other words are harder to learn.

Parent-infant observation for prediction of later childhood psychopathology in community-based samples: A systematic review

BACKGROUND

Difficulties in parent-child interaction are easily observed and are a potential target for early intervention. This review aimed to assess the utility of current observational methods used to assess parent-child interactions-within the first year of life-and their ability to screen and identify children from low-risk samples most at risk of developing childhood psychopathology.

METHODS

Six bibliographic databases were searched, and reference lists screened. All peer reviewed papers studying the association between an independent observation of parent-child interaction and later childhood psychopathology in community-based samples were included. Included studies were those recruiting from population or community-based birth cohort data, which we define as 'low-risk'. Studies based on populations known to have a diagnosis of psychiatric illness or developmental disorder, or at high genetic or environmental risk of being diagnosed with such disorder, were excluded. Results were synthesised qualitatively due to high heterogeneity.

RESULTS

20,051 papers were identified, nine were included in this study. Childhood psychopathology was associated with fewer positive parent-infant interactions, lower parent vocalisation frequency and lower levels of adult speech and activity. Maternal sensitivity was inversely related to separation anxiety and oppositional defiant/conduct disorders were associated with lower shared look rates. Disruptive behaviour disorders were associated with higher frequency of child vocalisation.

CONCLUSION

Assessment of parent-child interactions, particularly the level of maternal activity, may be an early indicator of later childhood psychopathology in low-risk samples. Further longitudinal, population-based studies are required.

TRIAL REGISTRATION

PROSPERO review registration: CRD42020162917 https://www.google.com/search?client=firefox-b-d&q=CRD42020162917.

Effect of sex and dyad composition on speech and gesture development of singleton and twin children

Children's early vocabulary shows sex differences - with boys having smaller vocabularies than age-comparable girls - a pattern that becomes evident in both singletons and twins. Twins also use fewer words than their singleton peers. However, we know relatively less about sex differences in early gesturing in singletons or twins, and also how singletons and twins might differ in their early gesture use. We examine the patterns of speech and gesture production of singleton and twin children, ages 0;10-to-3;4, during structured parent-child play. Boys and girls - singleton or twin - were similar in speech and gesture production, but singletons used a greater amount and diversity of speech and gestures than twins. There was no effect of twin dyad type (boy-boy, girl-girl, boy-girl) on either speech or gesture production. These results confirm earlier research showing close integration between gesture and speech in singletons in early language development, and further extend these patterns to twin children.

Does Early Child Language Predict Internalizing Symptoms in Adolescence? An Investigation in Two Birth Cohorts Born 30 Years Apart

Language is vital for social interaction, leading some to suggest early linguistic ability paves the way for good adolescent mental health. The relation between age-5 vocabulary and adolescent internalizing symptoms was examined in two U.K. birth cohorts that are nationally representative in terms of sex, ethnicity, and socioeconomic status: the 1970 British Cohort Study (BCS; N = 11,640) and the Millennium Cohort Study (MCS born ~2001; N = 14,754). In the BCS, no relation between receptive vocabulary and age-16 self-reported symptoms was observed (β = 0.00 [-0.03; 0.03]). In the MCS, better expressive vocabulary was associated with more age-14 self-reported symptoms (β = 0.05 [0.02; 0.07]). The direction of this effect was reversed for parent-reported symptoms. All effect sizes were small. The relation between childhood vocabulary and internalizing symptoms varies by generation and reporter.

Late Language Emergence: A literature review

Infants usually say their first word at the age of 12 months; subsequently, within the next 6-12 months, they develop a vocabulary of approximately 50 words, along with the ability to make two-word combinations. However, late talkers (LTs) demonstrate delayed speech in the absence of hearing impairments, cognitive developmental issues or relevant birth history. The prevalence of late language emergence (LLE) in toddlers is reported to be 10-15%. Studies of LTs are both theoretically and clinically significant. Early diagnosis and clinical intervention may result in relatively stable speech capabilities by the early school years. The present article aimed to review both theoretical and empirical studies regarding LLE within the process of first language acquisition, as well as methods for the early diagnosis of delayed speech in children and the authors' own clinical and theoretical recommendations.

The mother during pregnancy and the puerperium: Detailed data abstracted from the clinical obstetric records of ALSPAC pregnancies

Background: When the Avon Longitudinal Study of Parents and Children (ALSPAC) was planned, it was assumed that the clinical obstetric data would be easily accessible from the newly developed National Health Service computerised ‘STORK’ system. Pilot studies, however, showed that, although fairly accurate in regard to aspects of labour and delivery, it was, at the time (1990-2), inadequate for identifying the full antenatal and postnatal details of clinical complications and treatments of the women in the Study. Methods: A scheme was therefore developed to train research staff to find and abstract relevant details from clinical records onto proformas designed for the purpose. Extracting such data proved very time consuming (up to six hours for complicated pregnancies) and consequently expensive. Funding for the enterprise was obtained piecemeal using specific focussed grants to extract data for subsamples of the Study, including a random sample to serve as controls. Results: To date, detailed records have been completed for 8369 pregnancies, and a further 5336 (13,705 in total) have complete details on specific prenatal areas, including serial measures of maternal blood pressure, proteinuria and weight. In this Data Note we describe the information abstracted from the obstetric medical records concerning the mother during pregnancy, labour, delivery and the first two weeks of the puerperium. Information abstracted relating to the fetus (including fetal monitoring, presentation, method of delivery) and neonate (signs of asphyxia, resuscitation, treatment and well-being) will be described in a further Data Note. Conclusions: These data add depth to ALSPAC concerning ways in which the signs and symptoms, procedures and treatments of the mother prenatally, intrapartum and postnatally, may impact on the long-term health and development of both mother and child. They augment the data collected from the mothers’ questionnaires and the ‘STORK’ digital hospital data.

Children With Persistent Versus Transient Early Language Delay: Language, Academic, and Psychosocial Outcomes in Elementary School

Purpose The objective of this study was to compare children with persistent versus transient preschool language delay on language, academic, and psychosocial outcomes in elementary school. Method Children with persistent language delay (n = 30), transient language delay (n = 29), and no language delay (controls; n = 163) were identified from a population-based sample of twins. They were compared on language skills, academic achievement, and psychosocial adjustment in kindergarten and Grades 1, 3, 4, and 6. Results Children with persistent language delay continued to show language difficulties throughout elementary school. Furthermore, they had academic difficulties, in numeracy, and psychosocial difficulties (attention-deficit/hyperactivity disorder behaviors, externalizing behaviors, peer difficulties) from Grade 1 to Grade 6. Children with transient language delay did not differ from controls on language and academic performance. However, they showed more externalizing behaviors in kindergarten and peer difficulties in Grade 1 than controls. Conclusion Difficulties at school age are widespread and enduring in those with persistent early language delay but appear specific to psychosocial adjustment in those with transient language delay.

Causal Pathways for Specific Language Impairment: Lessons From Studies of Twins

Purpose This review article summarizes a program of longitudinal investigation of twins' language acquisition with a focus on causal pathways for specific language impairment (SLI) and nonspecific language impairment in children at 4 and 6 years with known history at 2 years. Method The context of the overview is established by legacy scientific papers in genetics, language, and SLI. Five recent studies of twins are summarized, from 2 to 16 years of age, with a longitudinal perspective of heritability over multiple speech, language, and cognitive phenotypes. Results Replicated moderate-to-high heritability is reported across ages, phenotypes, full population estimates, and estimates for clinical groups. Key outcomes are documentation of a twinning effect of risk for late language acquisition in twins that persists through 6 years of age, greater for monozygotic than dizygotic twins (although zygosity effects disappear at 6 years); heritability is greater for grammar and morphosyntax than other linguistic dimensions, from age 2 years through age 16 years, replicated within twin samples at subsequent age levels and across twin samples at age 16 years. Conclusion There is consistent support for legacy models of genetic influences on language acquisition, updated with a more precise growth signaling disruption model supported by twin data, as well as singleton data of children with SLI and nonspecific language impairment. Presentation Video https://doi.org/10.23641/asha.13063727.

Is there something special about twin families? A comparison of parenting styles in twin and non-twin families

Twin comparisons offer a powerful quasi-experimental design to study the impact of the family of origin on children's life chances. Yet, there are concerns about the generalizability of results obtained from twin studies because twin families are structurally different and twins have a genetic resemblance. We examine these concerns by comparing mothers' reports on their parenting styles for twin and non-twin children between twin and non-twin families, as well as within twin families. We use two German studies for our comparisons: TwinLife and pairfam. Our results demonstrate that twins receive more differential treatment and more emotional warmth than non-twins; however, these differences are largely accounted for by age differences between children. Overall, our results indicate that results on parenting obtained from twin studies can be generalized to non-twin families.

Neurodevelopmental outcome of preterm twins at 5 years of age

BACKGROUND

Twins are considered to be at an increased risk for perinatal mortality and morbidities, but it is unclear whether preterm twins are at an increased risk for poor developmental outcomes when compared to preterm singletons. Our aim was to compare the neurodevelopmental outcome of preterm twins vs singletons at 5 years of age.

METHODS

Very low birth weight and very low gestational age infants (twins n = 66, singletons n = 157) were recruited as a part of the PIPARI project in the Turku University Hospital, covering a regional population. Cognitive development, neuropsychological performance, and neurodevelopmental impairments (including cerebral palsy, hearing deficit, visual impairment, and intellectual disability) were evaluated at 5 years of age.

RESULTS

Twins and singletons had otherwise similar perinatal background factors, except for the higher proportion of preterm rupture of membranes in singletons. Twins had cognitive and neuropsychological outcomes that were otherwise comparable with singletons, but they had a slightly lower verbal intelligence quotient (estimate -5.81, 95% CI -11.14 to -0.48, p = 0.03). Being a twin was not a risk for neurodevelopmental impairments.

CONCLUSIONS

Our study shows that, contrary to a common hypothesis, the overall neurodevelopment of very preterm twins does not significantly differ from that of preterm singletons.

Heritability of Specific Language Impairment and Nonspecific Language Impairment at Ages 4 and 6 Years Across Phenotypes of Speech, Language, and Nonverbal Cognition

Purpose Early language and speech acquisition can be delayed in twin children, a twinning effect that diminishes between 4 and 6 years of age in a population-based sample. The purposes of this study were to examine how twinning effects influence the identification of children with language impairments at 4 and 6 years of age, comparing children with specific language impairment (SLI) and nonspecific language impairment (NLI); the likelihood that affectedness will be shared within monozygotic versus dizygotic twin pairs; and estimated levels of heritability for SLI and NLI. Twinning effects are predicted to result in elevated rates of language impairments in twins. Method The population-based twin sample included 1,354 children from 677 twin pairs, 214 monozygotic and 463 dizygotic, enrolled in a longitudinal study. Nine phenotypes from the same comprehensive direct behavioral assessment protocol were investigated at 4 and 6 years of age. Twinning effects were estimated for each phenotype at each age using structural equation models estimated via diagonally weighted least squares. Heritabilities were calculated for SLI and NLI. Results As predicted, the twinning effect increased the percentage of affected children in both groups across multiple language phenotypes, an effect that diminished with age yet was still not aligned to singleton age peers. Substantial heritability estimates replicated across language phenotypes and increased with age, even with the most lenient definition of affectedness, at -1 SD. Patterns of outcomes differed between SLI and NLI groups. Conclusions Nonverbal IQ is not on the same causal pathway as language impairments. Twinning effects on language acquisition affect classification of 4- and 6-year-old children as SLI and NLI, and heritability is most consistent in the SLI group. Clinical practice requires monitoring language acquisition of twins to avoid misdiagnosis when young or a missed diagnosis of language impairments at school entry.

Twins' and Singletons' Linguistic Environment: A Systematic Review

Background: Among twins, lower linguistic skills emerged when compared with singletons. Considering the association found between parental linguistic input and children's language development, exploring the differences between twins and singletons' linguistic environments could find variables that are potentially associated with the lower linguistic skills of twins.

Aim: The current systematic review aims to analyze and systematize the existing literature focused on the comparison of twins' and singletons' linguistic environments within their first 3 years of life. Methodological issues (i.e., the procedure used to assess the linguistic environment, the coding of the linguistic environment's features, the computational method employed to assess the parental linguistic input, and participant characteristics) and differences found among twins and singletons regarding their linguistic environment (i.e., linguistic input quantity, linguistic input complexity, linguistic features of child-directed speech, parental responsiveness, and directiveness, joint attention, and book reading) were highlighted.

Method: The Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) statement was followed. Eligible studies were searched through EBSCO, PubMed, and Web of Science. From this search, 1,347 study results emerged, and 8 studies were included.

Results: To our knowledge, this is the first systematic review focused on the comparison of twins' and singletons' linguistic environments. Differences between the groups were found in all of the included studies. Data against twins were generally identified regarding all the considered linguistic environment's features. However, conflicting results within and between the included studies emerged, mainly according to the computational method employed (i.e., twin moms value, twin direct dyadic value, twin direct dyadic + both value, and input directed toward both children simultaneously).

Conclusion: The disadvantaged linguistic environment of twins is likely due to limited parental resources and demands associated with the management of two children of the same age. However, the limited and conflicting data found did not allow for a firm conclusion to be drawn on the differences in the twins' and singletons' linguistic environments. Further studies on the topic are needed.

Longitudinal Study of Expressive Language and Speech of Twins at 3 and 5 Years: Outgrowing a Twinning Effect

Purpose This study examines the expressive language and speech of twins, relative to singletons, at 3 and 5 years, with the aim of determining if a twinning effect occurs during this developmental period. The possibility of twins outgrowing a twinning effect was investigated. Method A weighted population-based sample of 185 twins and 1,309 closely spaced singletons who participated in the Growing Up in Ireland study was analyzed. Their development was compared using a standardized expressive vocabulary assessment and parent interview at 3 and 5 years as well as teacher reports at 5 years. Causal steps mediation analyses were subsequently conducted to determine if gestation and birth weight mediated twin-singleton differences in expressive vocabulary development. Results Twins' expressive vocabulary scores were slightly lower than those of singletons at 3 years, but there was no significant difference at 5 years. More parents of twins reported "a little" concern about their child's expressive language and speech skills at 3 years, although there was no significant twin-singleton difference 2 years later. Teachers' ratings of twins' and singletons' expressive language and speech skills at 5 years were comparable. Shorter gestations and lower birth weights mediated twins' lower expressive vocabulary scores at 3 years. Conclusions A very small twinning effect on expressive vocabulary development exists at 3 years and is outgrown by 5 years. Twins born after shorter gestations or at a lower birth weight are at an increased risk of having less developed expressive vocabulary skills at 3 years.

Hearing, speech reception, vocabulary and language: population epidemiology and concordance in Australian children aged 11 to 12 years and their parents

OBJECTIVES

To describe the epidemiology and parent-child concordance of hearing, speech reception, vocabulary and language in Australian parent-child dyads at child age 11 to 12 years.

DESIGN

Population-based cross-sectional study (Child Health CheckPoint) nested within the Longitudinal Study of Australian Children.

SETTING

Assessment centres in seven Australian cities and eight regional towns or home visits around Australia, February 2015 to March 2016.

PARTICIPANTS

Of all participating CheckPoint families (n=1874), 1516 children (50% female) and 1520 parents (87% mothers, mean age 43.8 years) undertook at least one of four measurements of hearing and language.

OUTCOME MEASURES

Hearing threshold (better ear mean of 1, 2 and 4 kHz) from pure-tone audiometry, speech reception threshold, receptive vocabulary, expressive and receptive languages using a sentence repetition task. Parent-child concordance was examined using Pearson's correlation coefficients and adjusted linear regression models. Survey weights and methods accounted for Longitudinal Study of Australian Children's complex sampling and stratification.

RESULTS

Children had a similar speech reception threshold to parents (children mean -14.3, SD 2.4; parents -14.9, SD 3.2 dB) but better hearing acuity (children 8.3, SD 6.3; parents 13.4, SD 7.0 decibels hearing level). Standardised sentence repetition scores were similar (children 9.8, SD 2.9; parents 9.1, SD 3.3) but, as expected, parents had superior receptive vocabularies. Parent-child correlations were higher for the cognitively-based language measures (vocabulary 0.31, 95% CI 0.26 to 0.36; sentence repetition 0.29, 95% CI 0.24 to 0.34) than the auditory measures (hearing 0.18, 95% CI 0.13 to 0.23; speech reception threshold 0.18, 95% CI 0.13 to 0.22). Mother-child and father-child concordances were similar for all measures.

CONCLUSIONS

We provide population reference values for multiple measures spanning auditory and verbal communication systems in children and mid-life adults. Concordance values aligned with previous twin studies and offspring studies in adults, in keeping with polygenic heritability that is modest for audition but around 60% for language by late childhood.

Prenatal and perinatal risks for late language emergence in a population-level sample of twins at age 2

BACKGROUND

Late Language Emergence (LLE) in the first two years of life is one of the most common parental concerns about child development and reasons for seeking advice from health professionals. LLE is much more prevalent in twins (38%) than singletons (20%). In studies of language development in twins without overt disability, adverse prenatal and perinatal environments have been reported to play a lesser role in the etiology of LLE than adverse postnatal environments. However, there is a lack of population-level evidence about prenatal and perinatal risk factors for LLE in twins. This study investigated the extent to which prenatal and perinatal risk factors were associated with LLE in a population-level sample of twins at age 2 without overt disability.

METHODS

The sample comprised 473 twin pairs drawn from a population sample frame comprising statutory notifications of all births in Western Australia (WA), 2000-2003. Twin pairs in which either twin had a known developmental disorder or exposure to language(s) other than English were excluded. Of the 946 twins, 47.9% were male. There were 313 dizygotic and 160 monozygotic twin pairs. LLE was defined as a score at or below the gender-specific 10th percentile on the MacArthur Communicative Development Inventories: Words and Sentences (CDI-WS) (Words Produced). Bivariate and multivariable logistic regression was used to investigate risk factors associated with LLE.

RESULTS

In the multivariable model, risk factors for LLE in order of decreasing magnitude were: Gestational diabetes had an adjusted odds ratio (aOR) of 19.5 (95% confidence interval (CI) 1.2, 313.1); prolonged TSR (aOR: 13.6 [2.0, 91.1]); multiparity (aOR: 7.6 [1.6, 37.5]), monozygosity (aOR: 6.9 [1.7, 27.9]) and fetal growth restriction (aOR: 4.6 [1.7, 12.7]). Sociodemographic risk factors (e.g., low maternal education, socioeconomic area disadvantage) were not associated with increased odds of LLE.

CONCLUSIONS

The results suggest that adverse prenatal and perinatal environments are important in the etiology of LLE in twins at age 2. It is important that health professionals discuss twin pregnancy and birth risks for delayed speech and language milestones with parents and provide ongoing developmental monitoring for all twins, not just twins with overt disability.

Longitudinal Study of Language and Speech of Twins at 4 and 6 Years: Twinning Effects Decrease, Zygosity Effects Disappear, and Heritability Increases

Purpose

This study investigates the heritability of language, speech, and nonverbal cognitive development of twins at 4 and 6 years of age. Possible confounding effects of twinning and zygosity, evident at 2 years, were investigated among other possible predictors of outcomes.

Method

The population-based twin sample included 627 twin pairs and 1 twin without a co-twin (197 monozygotic and 431 dizygotic), 610 boys and 645 girls, 1,255 children in total. Nine phenotypes from the same comprehensive direct behavioral assessment protocol were investigated at 4 and 6 years of age. Twinning effects were estimated for each phenotype at each age using general linear mixed models using maximum likelihood.

Results

Twinning effects decreased from 4 to 6 years; zygosity effects disappeared by 6 years. Heritability increased from 4 to 6 years across all 9 phenotypes, and the heritability estimates were higher than reported previously, in the range of .44-.92 at 6 years. The highest estimate, .92, was for the clinical grammar marker.

Conclusions

Across multiple dimensions of speech, language, and nonverbal cognition, heritability estimates are robust. A finiteness marker of grammar shows the highest inherited influences in this early period of children's language acquisition.

Late Talkers: A Population-Based Study of Risk Factors and School Readiness Consequences

PURPOSE

This study was designed to (a) identify sociodemographic, pregnancy and birth, family health, and parenting and child care risk factors for being a late talker at 24 months of age; (b) determine whether late talkers continue to have low vocabulary at 48 months; and (c) investigate whether being a late talker plays a unique role in children's school readiness at 60 months.

METHOD

We analyzed data from the Early Childhood Longitudinal Study, a population-based sample of 9,600 children. Data were gathered when the children were 9, 24, 48, and 60 months old.

RESULTS

The risk of being a late talker at 24 months was significantly associated with being a boy, lower socioeconomic status, being a nonsingleton, older maternal age at birth, moderately low birth weight, lower quality parenting, receipt of day care for less than 10 hr/week, and attention problems. Being a late talker increased children's risk of having low vocabulary at 48 months and low school readiness at 60 months. Family socioeconomic status had the largest and most profound effect on children's school readiness.

CONCLUSIONS

Limited vocabulary knowledge at 24 and 48 months is uniquely predictive of later school readiness. Young children with low vocabularies require additional supports prior to school entry.

Developmental and behavioral outcomes of uncomplicated monochorionic diamniotic twins born in the third trimester

BACKGROUND

Relatively little is known about the neurodevelopmental and behavioral outcomes of monochorionic diamniotic (MCDA) twin pregnancies where there are no antenatal complications peculiar to monochorionicity or prematurity.

METHODS

Twenty-two MCDA twins (44 children) with an average age of 4.3 years, and with no antenatal complications detected by 28 weeks of gestation, were recruited from a feto-maternal unit database. Parents completed a battery of neurodevelopmental and behavioral assessment questionnaires.

RESULTS

Eighteen children (41%) were identified as having developmental or behavioral concerns, predominantly of mild severity, which in turn were associated with a lower birth weight of medium effect size (Cohen's d=0.59).

CONCLUSION

MCDA twins delivered in the third trimester with no antenatal monochorionic complications in the first two trimesters appear to be at risk for subtle neurodevelopmental difficulties, associated with a lower birth weight. Ongoing developmental surveillance of these children during preschool-age is indicated for early identification and intervention.

Less is More in Hong Kong: Investigation of Biscriptal and Trilingual Development Among Chinese Twins in a (Relatively) Small City

One salient characteristic of twin studies and the related behavioral genetics paradigm is the requirement of a large sample size. Countries or regions that are large in size and highly populated are at an advantage when implementing twin studies. However, given the fascinating and promising results obtained from twin studies, many researchers based in smaller countries or regions may still want to conduct twin studies in order to address local and theoretical issues. In this article, we have outlined the development of twin studies in Hong Kong, one of the Special Administrative Regions of China. The historical development and design of the two major twin studies of language and reading development implemented within Hong Kong are discussed, providing insights to researchers who also aspire to conduct twin studies in small regions.

Stress and coping in parents of newly born twins

OBJECTIVE

Research indicates that parents of twins have poorer psychosocial outcomes than parents of singletons. Parents who have conceived using assisted reproductive technology (ART) have been found to be at higher risk of negative psychosocial outcomes compared to parents who have conceived spontaneously. The current study aimed to model the factors associated with parenting stress of newly-born twins, using the Transactional Model of Stress.

METHODS

Data were collected using a cross-sectional survey design with participants identified from delivery records across Northern Ireland. Mothers and fathers (n = 104) of twins aged between 1 and 12 months old returned a questionnaire pack containing the Parenting Stress Index, Impact on the Family Scale-Financial Burden, Coping Orientation to Problems Experienced - Brief Version, Multidimensional Scale of Perceived Social Support, General Health Questionnaire and a demographic questionnaire.

RESULTS

There were no differences on psychological outcomes between parents who had conceived via ART and those who conceived spontaneously. Regression analyses found that social interaction and support is an important variable in terms of the psychological outcomes experienced by parents of twins.

CONCLUSION

Parents of newly-born twins regardless of the mode of conception should be considered an at risk group for parental distress. Support groups such as the Twins and Multiple Births Association could be important in providing that crucial social interaction and support that seems to be important in the emotional well-being of parents of twins.

Genetic and Environmental Links Between Natural Language Use and Cognitive Ability in Toddlers

Although the phenotypic correlation between language and nonverbal cognitive ability is well-documented, studies examining the etiology of the covariance between these abilities are scant, particularly in very young children. The goal of this study was to address this gap in the literature by examining the genetic and environmental links between language use, assessed through conversational language samples, and nonverbal cognition in a sample of 3-year-old twins (N = 281 pairs). Significant genetic and nonshared environmental influences were found for nonverbal cognitive ability and language measures, including mean length of utterance and number of different words, as well as significant genetic covariance between cognitive ability and both language measures.

Etiological Subgroups of Small-for-Gestational-Age: Differential Neurodevelopmental Outcomes

Objectives

It remains unclear why substantial variations in neurodevelopmental outcomes exist within small-for-gestational-age (SGA) children. We prospectively compared 5-y neurodevelopmental outcomes across SGA etiological subgroups.

Methods

Children born SGA (N = 1050) from U.S. Early Childhood Longitudinal Study-Birth Cohort (2001–2007) was divided into etiological subgroups by each of 7 well-established prenatal risk factors. We fit linear regression models to compare 5-y reading, math, gross motor and fine motor scores across SGA subgroups, adjusting for socio-demographic confounders.

Results

Compared to singleton SGA subgroup, multiple-birth SGA subgroup had lower mean reading (adjusted mean difference, -4.08 [95% confidence interval, -6.10, -2.06]) and math (-2.22 [-3.61, -0.84]) scores. These disadvantages in reading and math existed only among multiple-birth SGA subgroup without ovulation stimulation (reading, -4.50 [-6.64, -2.36]; math, -2.91 [-4.37, -1.44]), but not among those with ovulation stimulation (reading, -2.33 [-6.24, 1.57]; math 0.63 [-1.86, 3.12]). Compared to singleton SGA subgroup without maternal smoking and inadequate gestational weight gain, singleton SGA subgroup with co-occurrence of maternal smoking and inadequate gestational weight gain (GWG) had lower mean reading (-4.81 [-8.50, -1.12]) and math (-2.95 [-5.51, -0.38]) scores. These differences were not mediated by Apgar score.

Conclusions

Multiple-birth SGA subgroups (vs. singleton SGA) or singleton SGA subgroup with co-occurrence of smoking and inadequate GWG (vs. singleton SGA subgroup without maternal smoking and inadequate gestational weight gain) have poorer cognitive development up to 5 y.

Supporting sleep in early care and education: an assessment of observed sleep times using a sleep practices optimality index

AIM

The aim was to investigate whether the sleep practices in early childhood education (ECE) settings align with current evidence on optimal practice to support sleep.

BACKGROUND

Internationally, scheduled sleep times are a common feature of daily schedules in ECE settings, yet little is known about the degree to which care practices in these settings align with the evidence regarding appropriate support of sleep.

METHODS

Observations were conducted in 130 Australian ECE rooms attended by preschool children (Mean=4.9years). Of these rooms, 118 had daily scheduled sleep times. Observed practices were scored against an optimality index, the Sleep Environment and Practices Optimality Score, developed with reference to current evidence regarding sleep scheduling, routines, environmental stimuli, and emotional climate. Cluster analysis was applied to identify patterns and prevalence of care practices in the sleep time.

RESULTS

Three sleep practices types were identified. Supportive rooms (36%) engaged in practices that maintained regular schedules, promoted routine, reduced environmental stimulation, and maintained positive emotional climate. The majority of ECE rooms (64%), although offering opportunity for sleep, did not engage in supportive practices: Ambivalent rooms (45%) were emotionally positive but did not support sleep; Unsupportive rooms (19%) were both emotionally negative and unsupportive in their practices.

CONCLUSIONS

Although ECE rooms schedule sleep time, many do not adopt practices that are supportive of sleep. Our results underscore the need for education about sleep supporting practice and research to ascertain the impact of sleep practices in ECE settings on children's sleep health and broader well-being.

Proceeding From Observed Correlation to Causal Inference: The Use of Natural Experiments

This article notes five reasons why a correlation between a risk (or protective) factor and some specified outcome might not reflect environmental causation. In keeping with numerous other writers, it is noted that a causal effect is usually composed of a constellation of components acting in concert. The study of causation, therefore, will necessarily be informative on only one or more subsets of such components. There is no such thing as a single basic necessary and sufficient cause. Attention is drawn to the need (albeit unobservable) to consider the counterfactual (i.e., what would have happened if the individual had not had the supposed risk experience). Fifteen possible types of natural experiments that may be used to test causal inferences with respect to naturally occurring prior causes (rather than planned interventions) are described. These comprise five types of genetically sensitive designs intended to control for possible genetic mediation (as well as dealing with other issues), six uses of twin or adoptee strategies to deal with other issues such as selection bias or the contrasts between different environmental risks, two designs to deal with selection bias, regression discontinuity designs to take into account unmeasured confounders, and the study of contextual effects. It is concluded that, taken in conjunction, natural experiments can be very helpful in both strengthening and weakening causal inferences.

A latent profile analysis of math achievement, numerosity, and math anxiety in twins

Underperformance in math is a problem with increasing prevalence, complex etiology, and severe repercussions. This study examined the etiological heterogeneity of math performance in a sample of 264 pairs of 12-year-old twins assessed on measures of math achievement, numerosity and math anxiety. Latent profile analysis indicated five groupings of individuals representing different patterns of math achievement, numerosity and math anxiety, coupled with differing degrees of familial transmission. These results suggest that there may be distinct profiles of math achievement, numerosity and anxiety; particularly for students who struggle in math.

Paternal postnatal depressive symptoms, infant sleeping and feeding behaviors, and rigid parental regulation: a correlational study

Paternal postnatal depression (PND) is now recognized as a serious and prevalent problem, associated with poorer well-being and functioning of all family members. Aspects of infant temperament, sleeping and feeding perceived by parents as problematic are associated with maternal PND, however, less is known about paternal PND. This study investigated depressive symptoms (Edinburgh postnatal depression scale (EPDS)) in 219 fathers of infants aged from 1 to 24 weeks (median 7.0 weeks). Infant predictor variables were sleeping problems, feeding problems and both mother and father reported temperament. Control variables were partner's support, other support and life events. Rigidity of parenting beliefs regarding infant regulation was also measured as a potential moderating factor. Infant feeding difficulties were associated with paternal depressive symptoms, subsuming the variance associated with both sleep problems and temperament. This relationship was not moderated by regulation beliefs. It was concluded that infant feeding is important to fathers. Fathers of infants with feeding difficulties may not be able to fulfill their idealized construction of involved fatherhood. Role incongruence may have an etiological role in paternal PND.

Language development of children born following intracytoplasmic sperm injection (ICSI) combined with assisted oocyte activation (AOA)

BACKGROUND

The effect of assisted reproduction technology (ART) on language development is still unclear. Moreover, different techniques are introduced at rapid pace and are not always accompanied by extensive follow-up programmes.

AIMS

To investigate the language development of 3-10-year-old children born following ART using intracytoplasmic sperm injection (ICSI) combined with assisted oocyte activation (AOA), which is a highly specialized technique applied in cases with a history of fertilization failure following conventional ICSI. Secondly, a comparison is made between the language development of singletons and twins.

METHODS & PROCEDURES

Twenty children, six boys and 14 girls, born following ICSI combined with AOA and older than 3 years were included in the study. The mean age of the children was 5;4 years (range = 3;1-10;4 years; SD = 1;8 years). Expressive and receptive language development were assessed using the Clinical Evaluation of Language Fundamentals (CELF-IV-NL) for children older than 5 years and the Reynell Developmental Language Scales (RTOS) for children younger than or equal to 5 years.

OUTCOMES & RESULTS

The mean total score for language ability (in percentiles) was 56.8 (SD = 33.6), which corresponds to normal language skills. Significantly higher scores were found for AOA singletons compared with twins. For the general language, none of the children scored within the clinical zone for language disability corresponding with a percentile lower than 5.

CONCLUSION & IMPLICATIONS

This study presents the first data concerning language outcome in 3-10-year-old children born following AOA. General language scores of the AOA children in this study are located within the normal ranges. The language development of singletons was significantly better compared with twins. Although the results are reassuring for language development, in future long-term follow-up studies in this population are necessary.

Association between parent-infant interactions in infancy and disruptive behaviour disorders at age seven: a nested, case-control ALSPAC study

BACKGROUND

Effective early intervention to prevent oppositional/conduct disorders requires early identification of children at risk. Patterns of parent-child interaction may predict oppositional/conduct disorders but large community-based prospective studies are needed to evaluate this possibility.

METHODS

We sought to examine whether the Mellow Parenting Observational System (MPOS) used to assess parent-infant interactions at one year was associated with psychopathology at age 7. The MPOS assesses positive and negative interactions between parent and child. It examines six dimensions: anticipation of child's needs, responsiveness, autonomy, cooperation, containment of child distress, and control/conflict; these are summed to produce measures of total positive and negative interactions. We examined videos from the Avon Longitudinal Study of Parents and Children (ALSPAC) sub-cohort who attended the 'Children in Focus' clinic at one year of age. Our sample comprised 180 videos of parent-infant interaction: 60 from infants who received a psychiatric diagnostic categorisation at seven years and 120 randomly selected controls who were group-matched on sex.

RESULTS

A negative association between positive interactions and oppositional/conduct disorders was found. With the exception of pervasive developmental disorders (autism), an increase of one positive interaction per minute predicted a 15% (95% CI: 4% to 26%) reduction in the odds of the infant being case diagnosed. There was no statistically significant relationship between negative parenting interactions and oppositional/conduct disorders, although negative interactions were rarely observed in this setting.

CONCLUSIONS

The Mellow Parenting Observation System, specifically low scores for positive parenting interactions (such as Responsiveness which encompasses parental warmth towards the infant), predicted later psychiatric diagnostic categorisation of oppositional/conduct disorders.

Do Twins Differ From Single-Born Children on Rates of Behavioral Difficulty in Early Childhood? A Study of Sibling Relationship Risk Factors

The hypothesis that twinning raises risk for behavioral difficulties in childhood is persistent, yet there is limited and inconsistent empirical evidence. Simple mean comparison without control for confounders provides data on prevalence rates but cannot provide knowledge about risk or etiology. To assess the effect of twin relationship on behavior, comparison of patterns of association with single-born siblings may be informative. Analyses of data from an Australian sample of twins and single-born children (N = 305, mean age 4 years 9 months, and a follow-up 12 months later) were undertaken. The outcome measure was the Strengths and Difficulties Questionnaire. Predictor and control measures were obtained from parent report on the sibling/co-twin relationship behavior, family demographics, and obstetric history. We assessed difference between twins and single-born children in two respects: (a) mean behavioral difficulties, and (b) patterns of association between sibling relationship and behavioral difficulties, controlling for confounders. Results showed no differences in mean levels of behavioral difficulties between twins and single-born siblings identifying the importance of statistical control for family and obstetric adversity. Differences in patterns of association were found; for twin children, conflict in their co-twin relationship predicted externalizing behaviors, while for single-born children conflict predicted internalizing behaviors. The findings of mean differences between twin and single-born children in social background, but not in behavioral difficulties, underscore the necessity of statistical control to identify risk associated with twinning compared with risk associated with family and obstetric background factors.

Testing alternative hypotheses regarding the association between behavioral inhibition and language development in toddlerhood

Studies have reported an inverse association between language development and behavioral inhibition or shyness across childhood, but the direction of this association remains unclear. This study tested alternative hypotheses regarding this association in a large sample of toddlers. Data on behavioral inhibition and expressive and receptive language abilities were collected from 816 twins at ages 14, 20, and 24 months. Growth and regression models were fit to the data to assess the longitudinal associations between behavioral inhibition and language development from 14 to 24 months. Overall, there were significant associations between behavioral inhibition and expressive language, and minimal associations with receptive language, indicating that the association is better explained by reticence to respond rather than deficient language development.

Late language emergence in 24 month twins: Heritable and increased risk for LLE in twins

Purpose

This study investigated the etiology of Late Language Emergence (LLE) in 24-month-old twins considering possible twinning, zygosity, gender, and heritability effects for vocabulary and grammar phenotypes.

Method

A population-based sample of 473 twin pairs participated. Multilevel modelling estimated means and variances of vocabulary and grammar phenotypes, controlling for familiality. Heritability was estimated with DeFries-Fulker regression, and variance components models, to determine effects of heritability, shared-environment, and non-shared environment.

Results

Twins had lower average language scores than norms for singleborn children, with lower average performance for MZ than DZ twins, and lower for boys than girls, although gender and zygosity did not interact. Gender did not predict LLE. Significant heritability was detected for vocabulary (.26) and grammar phenotypes (.52/.43 for males/females) in the full sample, and in the sample selected for LLE (.42 and .44). LLE and the appearance of Word Combinations were also significantly heritable (.22-.23).

Conclusion

The findings revealed an increased likelihood of LLE in twin toddlers compared to singleborn children that is modulated by zygosity and gender differences. Heritability estimates are consistent with previous research for vocabulary and add further suggestion of heritable differences in early grammar acquisition.

Beliefs about infant regulation, early infant behaviors and maternal postnatal depressive symptoms

BACKGROUND

Young infants may have irregular sleeping and feeding patterns. Such regulation difficulties are known correlates of maternal depressive symptoms. Parental beliefs about their role in regulating infant behaviors also may play a role. We investigated the association of depressive symptoms with infant feeding/sleeping behaviors, parent regulation beliefs, and the interaction of the two.

METHOD

In 2006, 272 mothers of infants aged up to 24 weeks completed a questionnaire about infant behavior and regulation beliefs. Participants were recruited from general medical practices and child health clinics in Brisbane, Australia. Depressive symptomology was measured using the Edinburgh Postnatal Depression Scale. Other measures were adapted from the ALSPAC study.

RESULTS

Regression analyses were run controlling for partner support, other support, life events, and a range of demographic variables. Maternal depressive symptoms were associated with infant sleeping and feeding problems but not regulation beliefs. The most important infant predictor was sleep behaviors with feeding behaviors accounting for little additional variance. An interaction between regulation beliefs and sleep behaviors was found. Mothers with high regulation beliefs were more susceptible to postnatal depressive symptoms when infant sleep behaviors were problematic.

CONCLUSION

Mothers of young infants who expect greater control are more susceptible to depressive symptoms when their infant presents challenging sleep behavior.

Impact of prematurity on language skills at school age

PURPOSE

The existing literature on language outcomes in children born prematurely focuses almost exclusively on standardized test scores rather than discourse-level abilities. The authors of this study looked longitudinally at school-age language outcomes and potential moderating variables for a group of twins born prematurely versus a control group of twins born at full term, analyzing both standardized test results and language sample data from the population-based Western Reserve Reading Project (WRRP; Petrill, Deater-Deckard, Thompson, DeThorne, & Schatschneider, 2006).

METHOD

Fifty-seven children born prematurely, at ≤32 weeks or <1,500 g, were compared with 57 children born at full term and were matched for age, gender, race, and parental education. Data included discourse-level language samples and standardized test results, collected at average ages 7, 8, and 10 years. The language samples were analyzed to yield a number of semantic and syntactic measures that were consolidated via factor analysis.

RESULTS

Regression models showed significant differences between the 2 groups for standardized test results, although the mean score for both groups fell in the normal range. For the discourse-level language measures, however, differences never reached statistical significance. Parental education was significantly associated with improved standardized test scores.

CONCLUSIONS

These findings suggest that in the absence of frank neurological impairment, sophisticated semantic and syntactic skills may be relatively intact in the discourse-level language of children born prematurely. Implications for assessment, particularly the potential role of attention and executive function in standardized testing tasks, are reviewed.

The relationship between twin language, twins' close ties, and social competence

This study investigated the relationship between twin language, twins' close ties, and social competence in a prospective longitudinal study. We hypothesized that twins whose tie is close would be more likely to develop a twin language, and these twins would be less likely to develop social competence. In addition, we hypothesize that some environmental factors, such as having an older sibling, preschool attendance, zygosity, and sex are also related to twin language, twins' close ties, and social competence. At baseline in 1999 a mailed questionnaire survey was conducted, and a follow-up questionnaire was distributed in 2004 among 958 mothers. As a result, 516 respondents returned the questionnaire (53.9%). In this study, we used 261 twin pairs aged from 6 to 12 years (school-age children) for analysis, excluding those with missing values. In the present study, we found that zygosity and sex were associated with twins' close ties. Having an older sibling and preschool attendance did not affect the twins' close tie, twin language, or social competence. One of the most important findings was that social competence was not affected directly by twins' close tie, but was affected when a twin language was found.

Neonatal and neurodevelopmental outcome of children aged 3-10 years born following assisted oocyte activation

Assisted oocyte activation (AOA) using a calcium ionophore has been used for more than a decade following intracytoplasmic sperm injection (ICSI) fertilization failure. However, since AOA does not mimic precisely the physiological fertilization process, concerns exist about its use in human assisted reproduction. This study assessed the neonatal and neurodevelopmental outcome of children aged ≥ 3 years who had been born following AOA in our centre. Twenty-one children participated in the study (81% response rate; mean age 63.6 ± 21.07 months). Neonatal data were collected via questionnaires. Neurodevelopmental outcome was tested using the Reynell Developmental Language Scales or Clinical Evaluation of Language Fundamentals, Wechsler Preschool and Primary Scale of Intelligence or Wechsler Intelligence Scale for Children, and the Movement Assessment Battery for Children III. Behaviour was scored by the Social Communication Questionnaire, the Child Behaviour Checklist and the Teachers Report Form. For all tests and questionnaires, the mean outcomes lay within the expected ranges. These are first data on the developmental outcome of AOA children. The high response rate and the robustness of the tests support the data, which are reassuring although still considered preliminary. Therefore, AOA should still be performed only in selected couples.

Twin birth: an additional risk factor for poorer quality maternal interactions with very preterm infants?

BACKGROUND

Twin birth can be considered an additional risk factor for poor interactions between mothers and their very preterm (VP; <32 weeks' gestation) infants.

AIMS

To explore if mothers of (VP) twins experience higher levels of stress than mothers of singletons and if mother-twin infant dyads experience poorer quality interactions.

METHOD

Mothers of VP twin infants (N=17) were closely matched to mothers of VP singleton infants (N=17). Mother-infant interaction was assessed before discharge from hospital and during a home visit at three months corrected age using the Nursing Child Assessment Teaching Scale (NCATS). Mothers' responsiveness to their infants was assessed using the Responsivity subscale of the Home Observation for Measurement of the Environment (HOME) and mothers completed the Parenting Stress Index short form (PSI-SF).

RESULTS

Mothers of twins had significantly lower HOME responsiveness scores (median 9 vs. 10) at three months corrected age and were more likely to have total PSI-SF scores in the clinical range (>90th percentile) compared to mothers of singletons (Fishers exact probability=0.05). Twin infants had lower mean Total Child Domain NCATS scores than singletons both at discharge (9.07 vs. 11.33) and at three months corrected age (13.18 vs. 15.71) indicating they were less responsive communicators.

CONCLUSIONS

VP twins present a greater challenge than singletons as their mothers experience high levels of parenting stress. Although mothers appear to compensate for twin infants' poorer clarity of cues in a structured, one to one task, mothers of twins were less responsive than mothers of singletons in an unstructured setting.

A multivariate twin study of early literacy in Japanese Kana

This first Japanese twin study of early literacy development investigated the extent to which genetic and environmental factors influence individual differences in prereading skills in 238 pairs of twins at 42 months of age. Twin pairs were individually tested on measures of phonological awareness, kana letter name/sound knowledge, receptive vocabulary, visual perception, nonword repetition, and digit span. Results obtained from univariate behavioral-genetic analyses yielded little evidence for genetic influences, but substantial shared-environmental influences, for all measures. Phenotypic confirmatory factor analysis suggested three correlated factors: phonological awareness, letter name/sound knowledge, and general prereading skills. Multivariate behavioral genetic analyses confirmed relatively small genetic and substantial shared environmental influences on the factors. The correlations among the three factors were mostly attributable to shared environment. Thus, shared environmental influences play an important role in the early reading development of Japanese children.

Behavior genetics and postgenomics

The science of genetics is undergoing a paradigm shift. Recent discoveries, including the activity of retrotransposons, the extent of copy number variations, somatic and chromosomal mosaicism, and the nature of the epigenome as a regulator of DNA expressivity, are challenging a series of dogmas concerning the nature of the genome and the relationship between genotype and phenotype. According to three widely held dogmas, DNA is the unchanging template of heredity, is identical in all the cells and tissues of the body, and is the sole agent of inheritance. Rather than being an unchanging template, DNA appears subject to a good deal of environmentally induced change. Instead of identical DNA in all the cells of the body, somatic mosaicism appears to be the normal human condition. And DNA can no longer be considered the sole agent of inheritance. We now know that the epigenome, which regulates gene expressivity, can be inherited via the germline. These developments are particularly significant for behavior genetics for at least three reasons: First, epigenetic regulation, DNA variability, and somatic mosaicism appear to be particularly prevalent in the human brain and probably are involved in much of human behavior; second, they have important implications for the validity of heritability and gene association studies, the methodologies that largely define the discipline of behavior genetics; and third, they appear to play a critical role in development during the perinatal period and, in particular, in enabling phenotypic plasticity in offspring. I examine one of the central claims to emerge from the use of heritability studies in the behavioral sciences, the principle of minimal shared maternal effects, in light of the growing awareness that the maternal perinatal environment is a critical venue for the exercise of adaptive phenotypic plasticity. This consideration has important implications for both developmental and evolutionary biology.

Motor development of Japanese twins in childhood as reported by mothers

OBJECTIVE

Twins tend to lag behind singletons in their motor development, though the causes of this appear to be complicated and are not yet fully understood. The present study was performed to clarify the factors associated with the motor development of twins.

METHODS

The twins in the first group consisted of school applicants, including 1131 twin pairs, and the second group consisted of children of members of several maternal associations, and included 951 pairs. All data were gathered by questionnaire survey, and six gross-motor development milestones were analyzed in individuals or pairs.

RESULTS

Relative to general population norms in Japan, twins tended to be delayed in reaching several motor development milestones. Stepwise regression analysis showed that of all the variables measured, gestational age was the most influential on all six milestones. Birthweight was also influential on most milestones. Other factors affecting motor development milestones were parity for 'sitting without support,' 'pulling up to a standing position,' and 'walking without support,' and zygosity for 'walking without support.' In these cases, longer gestational age, heavier birthweighf, primiparity, and dizygosity showed a tendency toward earlier attainment of development. Motor development within pairs was more similar in monozygotic pairs compared with dizygotic pairs regarding each item, suggesting genetic contributions.

CONCLUSION

The results of the present study showed specific and nonspecific factors that affect motor development of twins. These findings should prove useful in understanding the motor development of twin children and help clinicians to refine maternal and child health care for multiple-birth children.

Relationship Between Parents' Report Rate of Twin Language and Factors Related to Linguistic Development: Older Sibling, Nonverbal Play and Preschool Attendance

The definition and nature of twin language has been a focus of recent studies concerned with the phenomenon. There has been a call for a tighter definition and understanding of the meaning of twin language (Thorpe et al., 2001). This article sought to identify social factors associated with the parent report of twin language and thus provide further understanding of the phenomenon. Data from 583 mothers of twins aged 25 to 59 months were analyzed using multiple logistic regression. Factors included in the modeling of parent-report twin language included social experience factors such as presence of siblings, attendance at preschool education and reports of nonverbal play. It was found that twin pairs who didn't have an older sibling, who showed frequent nonverbal play and who didn't attend preschool were more likely to have a twin language. Moreover, in the group not having an older sibling, the influence of whether twins attended preschool or not was strong and the odds ratio was 0.589 (95% confidence intervals 0.360–0.963). The findings suggest that social experience factors are important predictors of the parent reporting of twin language.