Early fetal growth: concepts and pitfalls

Unveiling sex bias and adverse neonatal outcomes in ultrasound estimation of gestational age: A population-based cohort study

BACKGROUND

Gestational age estimation by second-trimester ultrasound biometry introduces systematic errors due to sex differences in early foetal growth, consequently increasing the risk of adverse neonatal outcomes. Ultrasound estimation earlier in pregnancy may reduce this bias.

OBJECTIVES

To investigate the distribution of sex ratio by gestational age and estimate the risk of adverse outcomes in male foetuses born early-term and female foetuses born post-term by first- and second-trimester ultrasound estimations.

METHODS

This population-based study compared two cohorts of births with gestational age based on first- and second-trimester ultrasound in the Medical Birth Registry of Norway between 2016 and 2020. We used a log-binomial regression model to estimate adjusted relative risk (RR) with 95% confidence interval (CI) for Apgar score <7 at 5 min, umbilical artery pH <7.05, neonatal intensive care unit (NICU) admission and respiratory morbidity in relation to foetal sex.

RESULTS

The sex ratio at birth in gestational weeks 36-43 showed less male predominance in pregnancies estimated in first compared to second trimester. Any adverse outcome was registered in 627 of 4470 male infants born in gestational weeks 37-38 and 618 of 6406 females born ≥41 weeks. Male infants born in weeks 37-38 had lower risk of NICU admission (RR 0.76, 95% CI 0.58, 0.99), Apgar score <7 at 5 min (RR 0.63, 95% CI 0.28, 1.41) and respiratory morbidity (RR 0.68, 95% CI 0.37, 1.25) in first- compared to second-trimester estimations. Female infants estimated in first trimester born ≥41 weeks had lower risk of umbilical artery pH <7.05, NICU admissions and respiratory morbidity; however, CIs were wide.

CONCLUSIONS

Early ultrasound estimation of gestational age may reduce the excess risk of adverse neonatal outcomes and highlight the role of foetal sex and the timing of ultrasound assessment in the clinical evaluation of preterm and post-term pregnancies.

Fetal growth velocity standards from the Fetal Growth Longitudinal Study of the INTERGROWTH-21st Project

BACKGROUND

Human growth is susceptible to damage from insults, particularly during periods of rapid growth. Identifying those periods and the normative limits that are compatible with adequate growth and development are the first key steps toward preventing impaired growth.

OBJECTIVE

This study aimed to construct international fetal growth velocity increment and conditional velocity standards from 14 to 40 weeks' gestation based on the same cohort that contributed to the INTERGROWTH-21st Fetal Growth Standards.

STUDY DESIGN

This study was a prospective, longitudinal study of 4321 low-risk pregnancies from 8 geographically diverse populations in the INTERGROWTH-21st Project with rigorous standardization of all study procedures, equipment, and measurements that were performed by trained ultrasonographers. Gestational age was accurately determined clinically and confirmed by ultrasound measurement of crown-rump length at <14 weeks' gestation. Thereafter, the ultrasonographers, who were masked to the values, measured the fetal head circumference, biparietal diameter, occipitofrontal diameter, abdominal circumference, and femur length in triplicate every 5 weeks (within 1 week either side) using identical ultrasound equipment at each site (4-7 scans per pregnancy). Velocity increments across a range of intervals between measures were modeled using fractional polynomial regression.

RESULTS

Peak velocity was observed at a similar gestational age: 16 and 17 weeks' gestation for head circumference (12.2 mm/wk), and 16 weeks' gestation for abdominal circumference (11.8 mm/wk) and femur length (3.2 mm/wk). However, velocity growth slowed down rapidly for head circumference, biparietal diameter, occipitofrontal diameter, and femur length, with an almost linear reduction toward term that was more marked for femur length. Conversely, abdominal circumference velocity remained relatively steady throughout pregnancy. The change in velocity with gestational age was more evident for head circumference, biparietal diameter, occipitofrontal diameter, and femur length than for abdominal circumference when the change was expressed as a percentage of fetal size at 40 weeks' gestation. We have also shown how to obtain accurate conditional fetal velocity based on our previous methodological work.

CONCLUSION

The fetal skeleton and abdomen have different velocity growth patterns during intrauterine life. Accordingly, we have produced international Fetal Growth Velocity Increment Standards to complement the INTERGROWTH-21st Fetal Growth Standards so as to monitor fetal well-being comprehensively worldwide. Fetal growth velocity curves may be valuable if one wants to study the pathophysiology of fetal growth. We provide an application that can be used easily in clinical practice to evaluate changes in fetal size as conditional velocity for a more refined assessment of fetal growth than is possible at present (https://lxiao5.shinyapps.io/fetal_growth/). The application is freely available with the other INTERGROWTH-21st tools at https://intergrowth21.tghn.org/standards-tools/.

Design and other methodological considerations for the construction of human fetal and neonatal size and growth charts

This paper discusses the features of study design and methodological considerations for constructing reference centile charts for attained size, growth, and velocity charts with a focus on human growth charts used during pregnancy. Recent systematic reviews of pregnancy dating, fetal size, and newborn size charts showed that many studies aimed at constructing charts are still conducted poorly. Important design features such as inclusion and exclusion criteria, ultrasound quality control measures, sample size determination, anthropometric evaluation, gestational age estimation, assessment of outliers, and chart presentation are seldom well addressed, considered, or reported. Many of these charts are in clinical use today and directly affect the identification of at‐risk newborns that require treatment and nutritional strategies.

This paper therefore reiterates some of the concepts previously identified as important for growth studies, focusing on considerations and concepts related to study design, sample size, and methodological considerations with an aim of obtaining valid reference or standard centile charts. We discuss some of the key issues and provide more details and practical examples based on our experiences from the INTERGROWTH‐21^st Project. We discuss the statistical methodology and analyses for cross‐sectional studies and longitudinal studies in a separate article in this issue.

Current Issues in the Development of Foetal Growth References and Standards

PURPOSE OF REVIEW

This paper discusses the current issues in the development of foetal charts and is informed by a scoping review of studies constructing charts between 2012 and 2018.

RECENT FINDINGS

The scoping review of 20 articles revealed that there is still a lack of consensus on how foetal charts should be constructed and whether an international chart that can be applied across populations is feasible. Many of these charts are in clinical use today and directly affect the identification of at risk newborns that require treatment and nutritional strategies. However, there is no agreement on important design features such as inclusion and exclusion criteria; sample size and agreement on definitions such as what constitutes a healthy population of pregnant women that can be used for constructing foetal standards.

SUMMARY

This paper therefore reiterates some of these current issues and the scoping review showcases the heterogeneity in the studies developing foetal charts between 2012 and 2018. There is no consensus on these pertinent issues and hence if not resolved will lead to continued surge of foetal reference and standard charts which will only exacerbate the current problem of not being able to make direct comparisons of foetal size and growth across populations.

Maternal and fetal characteristics affect discrepancies between pregnancy-dating methods: a population-based cross-sectional register study

INTRODUCTION

Gestational age is estimated by ultrasound using fetal size as a proxy for age, although variance in early growth affects reliability. The aim of this study was to identify characteristics associated with discrepancies between last menstrual period-based (EDD-LMP) and ultrasound-based (EDD-US) estimated delivery dates.

MATERIAL AND METHODS

We identified all singleton births (n = 1 201 679) recorded in the Swedish Medical Birth Register in 1995-2010, to assess the association between maternal/fetal characteristics and large negative and large positive discrepancies (EDD-LMP earlier than EDD-US and 10th percentile in the discrepancy distribution vs. EDD-LMP later than EDD-US and 90th percentile). Analyses were adjusted for age, parity, height, body mass index, smoking, and employment status.

RESULTS

Women with a body mass index >40 kg/m² had the highest odds for large negative discrepancies (-9 to -20 days) [odds ratio (OR) 2.16, 95% CI 2.01-2.33]. Other factors associated with large negative discrepancies were: diabetes, young maternal age, multiparity, body mass index between 30 and 39.9 kg/m² or <18.5 kg/m² , a history of gestational diabetes, female fetus, shorter stature (<-1 SD), a history of preeclampsia, smoking or snuff use, and unemployment. Large positive discrepancies (+4 to +20 days) were associated with male fetus (OR 1.80, 95% CI 1.77-1.83), age ≥30 years, multiparity, not living with a partner, taller stature (>+1 SD), and unemployment.

CONCLUSIONS

Several maternal and fetal characteristics were associated with discrepancies between dating methods. Systematic associations of discrepancies with maternal height, fetal sex, and partly obesity, may reflect an influence on the precision of the ultrasound estimate due to variance in early growth.

Effects of ultrasound pregnancy dating on neonatal morbidity in late preterm and early term male infants: a register-based cohort study

Background

Assessing gestational age by ultrasound can introduce a systematic bias due to sex differences in early growth.

Methods

This cohort study included data on 1,314,602 births recorded in the Swedish Medical Birth Register. We compared rates of prematurity-related adverse outcomes in male infants born early term (gestational week 37–38) or late preterm (gestational week 35–36), in relation to female infants, between a time period when pregnancy dating was based on the last menstrual period (1973–1978), and a time period when ultrasound was used for pregnancy dating (1995–2010), in order to assess the method’s influence on outcome by fetal sex.

Results

As expected, adverse outcomes were lower in the later time period, but the reduction in prematurity-related morbidity was less marked for male than for female infants. After changing the pregnancy dating method, male infants born early term had, in relation to female infants, higher odds for pneumothorax (Cohort ratio [CR] 2.05; 95 % confidence interval [CI] 1.33–3.16), respiratory distress syndrome of the newborn (CR 1.99; 95 % CI 1.33–2.98), low Apgar score (CR 1.26; 5 % CI 1.08–1.47), and hyperbilirubinemia (CR 1.12; 95 % CI 1.06–1.19), when outcome was compared between the two time periods. A similar trend was seen for late preterm male infants.

Conclusion

Misclassification of gestational age by ultrasound, due to size differences, can partially explain currently reported sex differences in early term and late preterm infants’ adverse neonatal outcomes, and should be taken into account in clinical decisions and when interpreting study results related to fetal sex.

Electronic supplementary material

The online version of this article (doi:10.1186/s12884-016-1129-z) contains supplementary material, which is available to authorized users.

Investigating Implausible Gestational Age and High Birthweight Combinations

BACKGROUND

Birth certificate data overestimate national preterm births because a high percentage of last menstrual period (LMP) dates have errors. Study goals were to determine: (i) To what extent errors in transfer of birthweight and LMP date from medical records to birth certificates contribute to implausibly high birthweight-for-gestational-age births; (ii) What percentage of implausible births would be resolved if the clinical estimate (CE) from birth certificates were used instead of LMP-based gestational age, and with what degree of certainty; and (iii) Of those not resolved, what percentage had a medical explanation.

METHODS

Medical records and birth certificates for all singleton infants with implausibly high birthweight-for-gestational-age based on LMP delivered in the Kaiser Permanente Northwest system in Oregon during 1998-2007 were examined. Percentages of implausible records resolved under various scenarios were calculated.

RESULTS

A total of 100 births with implausibly high birthweight-for-gestational age combinations were identified. When LMP date and birthweight from medical records were used instead of from birth certificates, 31% of births with implausible combinations were resolved. Substituting the CE on the birth certificate for the LMP date resolved 92%. Of the latter, the clinician's gestational age estimate in the medical record was obtained in early pregnancy in 72%. Five of the eight births with unresolved implausible combinations were to mothers with diabetes; the remaining three had no documented medical explanation.

CONCLUSIONS

In this study, use of the birth certificate CE rather than the LMP resulted in a clinically reliable reclassification for the majority of implausible birthweight-for-gestational age deliveries.

International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown-rump length in the first trimester of pregnancy

OBJECTIVES

There are no international standards for relating fetal crown-rump length (CRL) to gestational age (GA), and most existing charts have considerable methodological limitations. The INTERGROWTH-21(st) Project aimed to produce the first international standards for early fetal size and ultrasound dating of pregnancy based on CRL measurement.

METHODS

Urban areas in eight geographically diverse countries that met strict eligibility criteria were selected for the prospective, population-based recruitment, between 9 + 0 and 13 + 6 weeks' gestation, of healthy well-nourished women with singleton pregnancies at low risk of fetal growth impairment. GA was calculated on the basis of a certain last menstrual period, regular menstrual cycle and lack of hormonal medication or breastfeeding in the preceding 2 months. CRL was measured using strict protocols and quality-control measures. All women were followed up throughout pregnancy until delivery and hospital discharge. Cases of neonatal and fetal death, severe pregnancy complications and congenital abnormalities were excluded from the study.

RESULTS

A total of 4607 women were enrolled in the Fetal Growth Longitudinal Study, one of the three main components of the INTERGROWTH-21(st) Project, of whom 4321 had a live singleton birth in the absence of severe maternal conditions or congenital abnormalities detected by ultrasound or at birth. The CRL was measured in 56 women at < 9 + 0 weeks' gestation; these were excluded, resulting in 4265 women who contributed data to the final analysis. The mean CRL and SD increased with GA almost linearly, and their relationship to GA is given by the following two equations (in which GA is in days and CRL in mm): mean CRL = -50.6562 + (0.815118 × GA) + (0.00535302 × GA(2) ); and SD of CRL = -2.21626 + (0.0984894 × GA). GA estimation is carried out according to the two equations: GA = 40.9041 + (3.21585 × CRL(0.5) ) + (0.348956 × CRL); and SD of GA = 2.39102 + (0.0193474 × CRL).

CONCLUSIONS

We have produced international prescriptive standards for early fetal linear size and ultrasound dating of pregnancy in the first trimester that can be used throughout the world.

Impact of pregnancy-associated malaria on infant malaria infection in southern Benin

BACKGROUND

Infants of mothers with placental Plasmodium falciparum infections at delivery are themselves more susceptible to malaria attacks or to infection in early life.

METHODOLOGY/ PRINCIPAL FINDINGS

To assess the impact of either the timing or the number of pregnancy-associated malaria (PAM) infections on the incidence of parasitemia or malaria attacks in infancy, we followed 218 mothers through pregnancy (monthly visits) up to delivery and their infants from birth to 12 months of age (fortnightly visits), collecting detailed clinical and parasitological data. After adjustment on location, mother's age, birth season, bed net use, and placental malaria, infants born to a mother with PAM during the third trimester of pregnancy had a significantly increased risk of infection (OR [95% CI]: 4.2 [1.6; 10.5], p = 0.003) or of malaria attack (4.6 [1.7; 12.5], p = 0.003). PAM during the first and second trimesters had no such impact. Similarly significant results were found for the effect of the overall number of PAM episodes on the time to first parasitemia and first malaria attack (HR [95% CI]: 2.95 [1.58; 5.50], p = 0.001 and 3.19 [1.59; 6.38], p = 0.001) respectively.

CONCLUSIONS/ SIGNIFICANCE

This study highlights the importance of protecting newborns by preventing repeated episodes of PAM in their mothers.

Systematic review of methodology used in ultrasound studies aimed at creating charts of fetal size

BACKGROUND

Reliable ultrasound charts are necessary for the prenatal assessment of fetal size, yet there is a wide variation of methodologies for the creation of such charts.

OBJECTIVE

To evaluate the methodological quality of studies of fetal biometry using a set of predefined quality criteria of study design, statistical analysis and reporting methods.

SEARCH STRATEGY

Electronic searches in MEDLINE, EMBASE and CINAHL, and references of retrieved articles.

SELECTION CRITERIA

Observational studies whose primary aim was to create ultrasound size charts for bi-parietal diameter, head circumference, abdominal circumference and femur length in fetuses from singleton pregnancies.

DATA COLLECTION AND ANALYSIS

Studies were scored against a predefined set of independently agreed methodological criteria and an overall quality score was given to each study. Multiple regression analysis between quality scores and study characteristics was performed.

MAIN RESULTS

Eighty-three studies met the inclusion criteria. The highest potential for bias was noted in the following fields: 'Inclusion/exclusion criteria', as none of the studies defined a rigorous set of antenatal or fetal conditions which should be excluded from analysis; 'Ultrasound quality control measures', as no study demonstrated a comprehensive quality assurance strategy; and 'Sample size calculation', which was apparent in six studies only. On multiple regression analysis, there was a positive correlation between quality scores and year of publication: quality has improved with time, yet considerable heterogeneity in study methodology is still observed today.

CONCLUSIONS

There is considerable methodological heterogeneity in studies of fetal biometry. Standardisation of methodologies is necessary in order to make correct interpretations and comparisons between different charts. A checklist of recommended methodologies is proposed.

Malaria in pregnancy: the difficulties in measuring birthweight

Recommendations for interventions to control malaria in pregnancy are often based on studies using birthweight as the primary endpoint. Differences in birthweight may be attributable partly to methodological difficulties. We performed a structured search of the literature using ‘malaria’, ‘pregnancy’ and ‘birth weight’ as search terms. Of the clinical trials reporting birthweight, only 33% (14/43) gave information about the timing of the measurement and details on the scales used. Seventy seven per cent explained how gestational age was estimated. We propose a standardised method for the measurement and reporting of birthweight in future studies.

Home pregnancy test compared to standard-of-care ultrasound dating in the assessment of pregnancy duration

OBJECTIVE

To determine the level of agreement between the Clearblue Digital Pregnancy Test with Conception Indicator home pregnancy test and standard-of-care ultrasound in assessing pregnancy duration in a real-life, observational setting encompassing routine, clinical care.

RESEARCH DESIGN AND METHODS

This was a prospective observational study of non-pregnant women seeking conception. Women collected daily urine samples from day 1 of their next menstrual cycle. If any volunteer became pregnant, daily urine samples continued to be collected for 43 days after the LH surge. Samples from day -7 to day +28 relative to the expected period (LH surge + 15 days) were tested using the home pregnancy test. This categorised any resulting pregnancies into one of three groups: 1-2 weeks, 2-3 weeks, and 3+ weeks since conception. Information from the standard UK ultrasound dating scan was also recorded by the midwife, including the expected delivery date according to ultrasound and the expected delivery date according to LMP.

MAIN OUTCOME MEASURES

Full data were available from 52 pregnant women who had conceived naturally. During the study analysis, 4786 urine samples were cross-compared with 52 routine 12-week NHS ultrasound assessments and the level of agreement between home pregnancy testing and standard-of-care ultrasound in determining pregnancy duration was calculated.

RESULTS

The agreement between the gestational age as calculated by the home pregnancy test result and the exact midwife-recorded gestational age using ultrasound was 82.3%. However, when a ± 5-day range was applied to the ultrasound reading (as per routine UK clinical practice), the level of agreement was 98%.

CONCLUSIONS

The home pregnancy test provides a significantly high (98%) level of agreement with standard-of-care ultrasound when assessing pregnancy duration in a real-life, observational setting which closely mirrors daily clinical practice.