Is the natural twinning rate still declining?

The Danish Twin Registry: An Updated Overview

The Danish Twin Registry (DTR) was established in the 1950s, when twins born from 1870 to 1910 were ascertained, and has since been extended to include twins from birth cohorts until 2009. The DTR currently comprises of more than 175,000 twins from the 140 birth cohorts. This makes the DTR the oldest nationwide twin register and among the largest in the world. The combination of data from several surveys, including biological samples and repeated measurements on the same individuals, and data from Danish national registers provides a unique resource for a wide range of twin studies. This article provides an updated overview of the data in the DTR: First, we provide a summary of the establishment of the register, the different ascertainment methods and the twins included; then follows an overview of major surveys conducted in the DTR since 1994 and a description of the DTR biobank, including a description of the molecular data created so far; finally, a short description is given of the linkage to Danish national registers at Statistics Denmark and some recent examples of studies using the various data resources in the DTR are highlighted.

Comparison of Late Mortality Among Twins Versus Singletons With Congenital Heart Defects

In 2014, in the United States, nearly 7% of newborns were twins. Congenital heart defects (CHDs) are more frequent in both monozygotic and dizygotic twins than in singletons. Still, the longer-term prognosis for CHD twins is unknown. Here we assess the mortality pattern for CHD twins up to age 36 years and compare it with that for non-CHD twins, non-CHD co-twins, and CHD singletons. We identified all twins and a 5% random sample of all singletons born in Denmark from 1977 to 2009 by linking Danish national population and health registers. CHD cases were defined as subjects having a primary inpatient diagnosis of CHD (excluding preterm ductus) within the first year of life, and mortality was assessed through 2013. Among 63,362 live-born twin individuals, a total of 373 twins (0.59%) had a CHD diagnosis, whereas the corresponding numbers for singletons were 383 of 98,647 (0.39%). During the follow-up, 82 (22.0%) CHD twins died compared with 91 (23.8%) CHD singletons (p = 0.56). Despite a 5 times higher proportion of prematurity, CHD twins had a tendency toward only a moderately increased neonatal mortality compared with CHD singletons (hazard ratio 1.5, 95% confidence interval 0.94 to 2.5), and after the neonatal period up to age 36 the tendency was reversed (hazard ratio 0.8, 95% confidence interval 0.5 to 1.2). A potential underlying mechanism for this mortality pattern is selective intrauterine and neonatal mortality of twins with the most severe CHD. In conclusion, the study indicates that the overall survival prognosis for CHD twins is similar to that of CHD singletons.

Academic performance of opposite-sex and same-sex twins in adolescence: A Danish national cohort study

Testosterone is an important hormone in the sexual differentiation of the brain, contributing to differences in cognitive abilities between males and females. For instance, studies in clinical populations such as females with congenital adrenal hyperplasia (CAH) who are exposed to high levels of androgens in utero support arguments for prenatal testosterone effects on characteristics such as visuospatial cognition and behaviour. The comparison of opposite-sex (OS) and same-sex (SS) twin pairs can be used to help establish the role of prenatal testosterone. However, although some twin studies confirm a masculinizing effect of a male co-twin regarding for instance perception and cognition it remains unclear whether intra-uterine hormone transfer exists in humans. Our aim was to test the potential influences of testosterone on academic performance in OS twins. We compared ninth-grade test scores and teacher ratings of OS (n=1812) and SS (n=4054) twins as well as of twins and singletons (n=13,900) in mathematics, physics/chemistry, Danish, and English. We found that males had significantly higher test scores in mathematics than females (.06-.15 SD), whereas females performed better in Danish (.33-.49 SD), English (.20 SD), and neatness (.45-.64 SD). However, we did not find that OS females performed better in mathematics than SS and singleton females, nor did they perform worse either in Danish or English. Scores for OS and SS males were similar in all topics. In conclusion, this study did not provide evidence for a masculinization of female twins with male co-twins with regard to academic performance in adolescence.

Twins and Their Health Cost: Consequences of Multiple Births on Parental Health and Mortality in Denmark and England and Wales

The rapid increase in twinning rates in developed countries has increased interest in the question as to whether twin mothers have higher mortality and more health problems than mothers of singletons. Here we use a national survey, the Office for National Statistics Longitudinal Study of England & Wales, and a linkage between the Danish Twin Registry and the Danish population register to examine mortality patterns after age 45 (50 for fathers) for twin parents and the whole population born from 1911 to 1950. For England and Wales, presence of limiting long-term illnesses and self-rated health status was also investigated. Overall similar health and mortality was found for twin parents and the whole population although both life table methods and survival analysis suggested a slight excess mortality among older cohorts of twin mothers in England and Wales.

The Danish Twin Registry in the New Millennium

The Danish Twin Registry is the oldest national twin register in the world, initiated in 1954, and, by the end of 2005, contained more than 75,000 twin pairs born in the between 1870 and 2004. The Danish Twin Registry is used as a source for studies on the genetic influence on normal variation in clinical parameters associated with the metabolic syndrome and cardiovascular diseases, clinical studies of specific diseases, and aging and age-related health problems. The combination of survey data, clinical data and linkage to national health-related registers enables follow-up studies of both the general twin population and twins from clinical studies. This paper summarizes the newest extension of the register and gives examples of new developments and phenotypes studied.

Coffee and Smoking as Risk Factors of Twin Pregnancies: The Danish National Birth Cohort

Twinning rates have changed substantially over time for reasons that are only partly known. In this study we studied smoking, coffee and alcohol intake, and their possible interaction with obesity as potential determinants of twinning rates using data from the Danish National Birth Cohort between 1996 and 2002. We identified 82,985 pregnancies: 81,954 singleton and 1031 twins. For the twins we had data to classify 121 as monozygotic, 189 dizygotic (same sex), 313 dizygotic (opposite sex) but, 408 were of the same sex but with unknown zygosity. All mothers were interviewed about their prepregnancy weight and height, coffee and alcohol intake, smoking habits, and potential confounding factors at early stages of pregnancy. We identified smoking (> 10 cigarettes/day) as a possible determinant of twinning, particularly for dizygotic twinning rates (same sex) and furthermore corroborated that obesity and the mother's age are strong correlates of twinning. Others have found coffee intake to increase twinning rates but that is not seen in these data.

Twinning Rates According to Maternal Birthweight

Dizygotic twinning rates have changed over time, which has been seen as a sign of a decline in fecundity. Since a woman's birthweight has been shown to be a marker of her fecundity, maternal birthweight may correlate with subsequent twinning rates. In the Danish National Birth Cohort (1996–2002), we examined if maternal birthweight, and whether she was born at term or preterm, correlated with her probability of multiple birth. For 20,719 live born infants, we had self-reported information about maternal birthweight, collected during the first wave of the 7-year follow-up, and information on multiple births from record linkage. The association between maternal birthweight and multiple births was investigated by use of logistic regression and presented as odds ratios (ORs) with 95% confidence intervals (CIs). Compared to women born at term with a birthweight of 3001–4000 g, women with a birthweight > 4500 g appeared to have higher chance of multiple birth while women with a birthweight of 4001–4500 had a lower chance, especially if the analysis was restricted to women with a BMI < 25 (Adjusted ORs 2.3 [1.0–5.4] and 0.4 [0.2–0.9] respectively). Odds ratios for dizygotic twinning were of the same magnitude. In women with a BMI ≥ 25, no obvious pattern was present. Our findings do not indicate that twinning is a fecundity indicator. Women with a birthweight that may indicate a pregnancy complicated with gestational diabetes had the highest rate of multiple birth. These findings are new and should be put to a critical test in other data sources.

Infertility, infertility treatment and twinning: the Danish National Birth Cohort

BACKGROUND

We have previously observed that an increasing time to pregnancy (TTP) is associated with a reduced frequency of twin deliveries in couples not receiving infertility treatment. By using updated information, we assessed the frequencies of dizygotic (DZ) and monozygotic (MZ) twin deliveries as a function of infertility (TTP > 12 months), as well as infertility treatment.

METHODS

From the Danish National Birth Cohort (1997-2003), we identified 51 730 fertile couples with TTP <or= 12 months, 5838 infertile couples who conceived naturally with TTP > 12 months and 5163 infertile couples who conceived after treatment. Information on zygosity, available for part of the cohort (1997-2000), was based on standardized questions on the similarities between the twins at the age of 3-5 years.

RESULTS

Compared with fertile couples, the frequency of DZ twin deliveries was lower for infertile couples conceiving naturally (odds ratio 0.4, 95% confidence interval 0.2-0.7) and was much higher for infertile couples conceiving after treatment (17.3, 14.4-20.7). The frequency of DZ twin deliveries decreased with TTP in untreated couples, whereas the frequency of MZ twin deliveries remained constant.

CONCLUSIONS

The frequency of DZ twin deliveries decreased with TTP and substantially increased with infertility treatment, whereas MZ twin deliveries remained substantially unchanged.

Twin pregnancy possibly associated with high semen quality

BACKGROUND

Recent studies found an association between a long waiting time to pregnancy (TTP) and reduced probability of twinning and a reduced dizygotic (DZ) twinning rate in subfertile men. However, it remains unsolved whether semen quality is associated with twin offspring. We therefore studied the semen quality in a group of fathers of naturally conceived twins.

METHODS

In this study, 37 fathers of DZ twins and 15 fathers of monozygotic (MZ) twins participated, and 349 normal fertile men served as a reference group. All men delivered a semen sample, underwent a physical examination and completed a questionnaire.

RESULTS

After adjustment, fathers of DZ and MZ twins had 3.6 (95% CI 1.7; 5.4) and 4.6 (95% CI 2.0; 7.2) percentage points higher percentage of sperm cells with normal morphologic features and percentages of motile sperm cells were 11.5 (95% CI 7.2; 15.9) and 12.5 (95% CI 6.3; 18.6) percentage points higher than the reference group (P < 0.01). Fathers of DZ twins and MZ twins had 24.7 (95% CI; -9.1; 71.3) and 17.0% (-25.2%; 83.0%) higher sperm concentration than the reference group.

CONCLUSIONS

Fathers of DZ twins had a better semen quality than the reference group, which supports the assumption that spontaneous DZ twinning rate can be used as a sensor of male fecundity of a population.

Comparison of academic performance of twins and singletons in adolescence: follow-up study

OBJECTIVES

To determine whether twins in recent cohorts show similar academic performance in adolescence to singletons and to test the effect of birth weight on academic performance in twins and singletons.

DESIGN

Follow-up study.

SETTING

Denmark.

PARTICIPANTS

All twins (n=3411) and a 5% random sample of singletons (n=7796) born in Denmark during 1986-8.

MAIN OUTCOME MEASURES

Test scores in ninth grade (age 15 or 16), birth weight, gestational age at birth, parents' age, and parents' education.

RESULTS

Ninth grade test scores were normally distributed, with almost identical mean and standard deviations for twins and singletons (8.02 v 8.02 and 1.05 v 1.06) despite the twins weighing on average 908 g (95% confidence interval 886 to 930 g) less than the singletons at birth. Controlling for birth weight, gestational age at birth, age at test, and parents' age and education confirmed the similarity of test scores for twins and singletons (difference 0.04, 95% confidence interval -0.03 to 0.10). A significant, positive association between test score and birth weight was observed in both twins and singletons, but the size of the effect was small: 0.06-0.12 standard deviations for every kilogram increase in birth weight.

CONCLUSIONS

Although older cohorts of twins have been found to have lower mean IQ scores than singletons, twins in recent Danish cohorts show similar academic performance in adolescence to that of singletons. Birth weight has a minimal effect on academic performance in recent cohorts; for twins this effect is best judged relative to what is a normal birth weight for twins and not for singletons.