Invited Commentary: Aligning Methodological Research on Pregnancy Weight Gain With the Questions That Matter Most for Public Health Guidelines

The inherent correlation between the total amount of weight gained in pregnancy and the duration of pregnancy creates major methodological challenges in the study of pregnancy weight gain. In this issue (Am J Epidemiol. 2022;191(10):1687-1699), Richards et al. examine the extent to which different measures of pregnancy weight gain (including covariate adjustment for gestational age and standardizing weight gain for gestational duration using a pregnancy weight gain chart) are able to disentangle the effects of low weight gain on perinatal health from the role of younger gestational age at delivery for 3 outcomes: small-for-gestational-age birth, cesarean delivery, and low birth weight. While methodological research to understand how to best disentangle the effects of gestational weight gain from pregnancy duration is valuable, we argue that the practical utility of this type of research would be increased by aligning the specific research questions more closely with health outcomes on which evidence is most needed-those not considered in current weight gain guidelines due to lack of high-quality evidence (such as pre-eclampsia and stillbirth). Further, evaluations of weight gain charts should separate out the potential for bias introduced by the use of a normative chart per se from the use of a chart unsuitable for the study population.

Body composition reference charts for infants from birth to 24 months: Multicenter Infant Body Composition Reference Study

BACKGROUND

Body composition assessment in the first 2 y of life provides important insights into child nutrition and health. The application and interpretation of body composition data in infants and young children have been challenged by a lack of global reference data.

OBJECTIVES

We aimed to develop body composition reference charts of infants aged 0-6 mo based on air displacement plethysmography (ADP) and those aged 3-24 mo based on total body water (TBW) by deuterium dilution (DD).

METHODS

Body composition was assessed by ADP in infants aged 0-6 mo from Australia, India, and South Africa. TBW using DD was assessed for infants aged 3-24 mo from Brazil, Pakistan, South Africa, and Sri Lanka. Reference charts and centiles were constructed for body composition using the lambda-mu-sigma method.

RESULTS

Sex-specific reference charts were produced for FM index (FMI), FFM index (FFMI), and percent FM (%FM) for infants aged 0-6 mo (n = 470 infants; 1899 observations) and 3-24 mo (n = 1026 infants; 3690 observations). When compared with other available references, there were observable differences but similar patterns in the trajectories of FMI, FFMI, and %FM.

CONCLUSIONS

These reference charts will strengthen the interpretation and understanding of body composition in infants across the first 24 mo of life.

Comparison between the Brazilian and 3 international gestational weight gain charts

BACKGROUND

Little is known about the ability of the recently released Brazilian gestational weight gain (GWG) charts to predict the occurrence of adverse birth outcomes.

OBJECTIVES

We compared the new Brazilian weight gain charts with 3 international charts and determined their ability to predict the occurrence of small-for-gestational-age (SGA) and large-for-gestational-age (LGA) births in Brazilian women.

METHODS

A subsample of 6888 adult women (43,931 weight measurements) with singleton pregnancies from a nationwide, hospital-based cohort study conducted in 2011-2012 was analyzed. Selected percentiles from Brazilian GWG charts were compared with those from American, International Fetal and Newborn Growth Consortium for the 21st Century study, and Lifecycle consortium charts. Sensitivity, specificity, and AUROC values for SGA and LGA births were estimated with 95% CIs using the classification of GWG below or above selected percentiles of each chart.

RESULTS

The weight gain corresponding to a given percentile varied among the charts, especially for women with pre-pregnancy overweight and obesity. The proportions of women with GWG classified below or above selected percentiles were closest to the expected values for all pre-pregnancy BMI categories in the Brazilian and Lifecycle charts. At the 10th percentile, the highest sensitivity for SGA births was observed for the American charts at midpregnancy (36.8%) and the highest specificity was observed using the Brazilian charts in the first trimester (93.4%). At the 90th percentile, the highest sensitivity for LGA births occurred in midpregnancy for the Lifecycle charts (26.8%) and the highest specificity occurred in the American charts using total GWG (97.1%). All the AUROCs were under 0.5 for SGA births and ranged from 0.55 (first trimester) to 0.62 (total GWG) for LGA births.

CONCLUSIONS

The charts differ in GWG trajectories, especially for women with overweight and obesity. The 4 charts had low predictive ability of SGA and LGA births and should not be considered as isolated screening tools for those outcomes.

Adverse neonatal outcome in twin pregnancy complicated by small-for-gestational age: twin vs singleton reference charts

OBJECTIVE

The use of twin-specific vs singleton growth charts in the assessment of twin pregnancy has been controversial. The aim of this study was to assess whether a diagnosis of small-for-gestational age (SGA) made using twin-specific estimated-fetal-weight (EFW) and birth-weight (BW) charts is associated more strongly with adverse neonatal outcomes in twin pregnancies, compared with when the diagnosis is made using singleton charts.

METHODS

This was a cohort study of twin pregnancies delivered at St George's Hospital, London, between January 2007 and May 2020. Twin pregnancies complicated by intrauterine death of one or both twins, fetal aneuploidy or major abnormality, twin-twin transfusion syndrome or twin anemia-polycythemia sequence and those delivered before 32 weeks' gestation, were excluded. SGA was defined as EFW or BW below the 10^th centile, and was assessed using both twin-specific and singleton EFW and BW charts. The main study outcome was composite adverse neonatal outcome. Mixed-effects logistic regression analysis with random pregnancy-level intercepts was used to test the association between SGA classified using the different charts and adverse neonatal outcome.

RESULTS

A total of 1329 twin pregnancies were identified, of which 913 (1826 infants) were included in the analysis. Of these pregnancies, 723 (79.2%) were dichorionic and 190 (20.8%) were monochorionic. Using the singleton charts, 33.3% and 35.7% of pregnancies were classified as SGA based on EFW and BW, respectively. The corresponding values were 5.9% and 5.6% when using the twin-specific charts. Classification as SGA based on EFW using the twin charts was associated significantly with composite adverse neonatal outcome (odds ratio (OR), 4.78 (95% CI, 1.47-14.7); P = 0.007), as compared with classification as appropriate-for-gestational age (AGA). However, classification as SGA based on EFW using the singleton standard was not associated significantly with composite adverse neonatal outcome (OR, 1.36 (95% CI, 0.63-2.88); P = 0.424). Classification as SGA based on EFW using twin-specific standards provided a significantly better model fit than did using the singleton standard (likelihood ratio test, P < 0.001). When twin-specific charts were used, classification as SGA based on BW was associated significantly with a 9.3 times increased odds of composite adverse neonatal outcome (OR, 9.27 (95% CI, 2.86-30.0); P < 0.001). Neonates classified as SGA according to the singleton BW standard but not according to the twin-specific BW standards had a significantly lower rate of composite adverse neonatal outcome than did AGA twins (OR, 0.24 (95% CI, 0.07-0.66); P = 0.009).

CONCLUSIONS

The singleton charts classified one-third of twins as SGA, both prenatally and postnatally. Infants classified as SGA according to the twin-specific charts, but not those classified as SGA according to the singleton charts, had a significantly increased risk of adverse neonatal outcome compared with infants classified as AGA. This study provides further evidence that twin-specific charts perform better than do singleton charts in the prediction of adverse neonatal outcome in twin pregnancies. The use of these charts may reduce misclassification of twins as SGA and improve identification of those that are truly growth restricted. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

A survey of current practice in reporting third trimester fetal biometry and Doppler in Australia and New Zealand

Introduction

Inconsistent reporting practices in third trimester ultrasound, the choice of reference charts in particular, have the potential to misdiagnose abnormal fetal growth. But this may lead to unnecessary anxiety and confusion amongst patients and clinicians and ultimately influence clinical management. Therefore, we sought to determine the extent of variability in choice of fetal biometry and Doppler reference charts and reporting practices in Australia and New Zealand.

Methods

Clinicians performing and/or reporting obstetric ultrasound were invited to answer questions about fetal biometry and Doppler charts in a web-based survey.

Results

At least four population-based charts are in current use. The majority of respondents (78%) report the percentile for known gestational age (GA) alongside measurements and 63% using a cut-off of estimated fetal weight (EFW) < 10^th percentile when reporting small for gestational age (SGA) and/or fetal growth restriction (FGR). The thresholds for the use of fetal and maternal Doppler in third trimester ultrasound varied in terms of the GA, EFW cut-off, and how measures were reported. The majority of respondents were not sure of which Doppler charts were used in their practice.

Conclusion

This survey revealed inconsistencies in choice of reference chart and reporting practices. The potential for misdiagnosis of abnormal fetal growth remains a significant issue.

Twin chorionicity-specific population birth-weight charts adjusted for estimated fetal weight

OBJECTIVES

To construct chorionicity-specific birth-weight reference charts for dichorionic diamniotic (DCDA) and monochorionic diamniotic (MCDA) twin pregnancies, incorporating estimated-fetal-weight (EFW) data in order to adjust for the relationship between suboptimal growth and preterm delivery. An additional aim was to determine if the inclusion of complicated twin pregnancies impacts on the reference charts produced.

METHODS

The inclusion criteria for this retrospective cohort study were twin pregnancy of known DCDA or MCDA chorionicity, known pregnancy outcome, last ultrasound scan within 14 days before birth and delivery between 25 and 38 weeks' gestation (Analysis A). An analysis was also conducted excluding pregnancies with complications recorded (Analysis B). Previously published twin EFW reference ranges were used in the analysis. A joint statistical model for EFW and observed birth weight for each pregnancy was created in order to estimate population birth-weight reference ranges corresponding to the distribution expected if all pregnancies delivered at any given gestational age. It was not assumed that the median EFW was equal to birth weight for any given gestational age. The models were fitted using a Bayesian approach.

RESULTS

We retrieved data on 1664 twin pregnancies, of which 707 DCDA and 241 MCDA pregnancies met the inclusion criteria. In Analysis A, the estimated population median birth weight was similar to the median EFW at around 27 weeks' gestation but fell below the EFW values with increasing gestation, being 156 g lower in both DCDA and MCDA pregnancies at 35 weeks; this finding was confirmed by direct comparison of the last EFW and birth-weight values in each pregnancy. When the analysis was repeated after excluding complicated twin pregnancies (Analysis B), compared with Analysis A, there was very little difference in the median birth-weight results obtained across gestation. The largest absolute difference between Analyses A and B for DCDA twins was at 31, 32 and 33 weeks, with a 9-g lower median birth weight in Analysis A compared with Analysis B. The largest absolute difference for MCDA twins was greater than that for DCDA twins, with a 21-g lower median birth weight at 25 weeks in Analysis A compared with Analysis B.

CONCLUSIONS

We have established population chorionicity-specific birth-weight reference charts for DCDA and MCDA twin pregnancies, corresponding to the range expected were all pregnancies to deliver at any given gestational age. In this population of twins, the median birth weight was consistently lower than that reported for singletons, and there was variation in the median birth weight at different gestational ages according to chorionicity. © 2021 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology. - Legal Statement: This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Perinatal Outcomes of Small for Gestational Age in Twin Pregnancies: Twin vs. Singleton Charts

Twin pregnancies are commonly assessed using singleton growth and birth weight reference charts. This practice has led to a significant number of twins labelled as small for gestational age (SGA), causing unnecessary interventions and increased risk of iatrogenic preterm birth. However, the use of twin-specific charts remains controversial. This study aims to assess whether twin-specific estimated fetal weight (EFW) and birth weight (BW) charts are more predictive of adverse outcomes compared to singleton charts. Centiles of EFW and BW were calculated using previously published singleton and twin charts. Categorical data were compared using Chi-square or McNemar tests. The study included 1740 twin pregnancies, with the following perinatal adverse outcomes recorded: perinatal death, preterm birth <34 weeks, hypertensive disorders of pregnancy (HDP) and admissions to the neonatal unit (NNU). Twin-specific charts identified prenatally and postnatally a smaller proportion of infants as SGA compared to singleton charts. However, twin charts showed a higher percentage of adverse neonatal outcomes in SGA infants than singleton charts. For example, perinatal death (SGA 7.2% vs. appropriate for gestational age (AGA) 2%, p < 0.0001), preterm birth <34 weeks (SGA 42.1% vs. AGA 16.4%, p < 0.0001), HDP (SGA 21.2% vs. AGA 13.5%, p = 0.015) and NNU admissions (SGA 69% vs. AGA 24%, p < 0.0001), when compared to singleton charts (perinatal death: SGA 2% vs. AGA 1%, p = 0.029), preterm birth <34 weeks: (SGA 20.6% vs. AGA 17.4%, p = 0.020), NNU admission: (SGA 34.5% vs. AGA 23.9%, p < 0.000). There was no significant association between HDP and SGA using the singleton charts (p = 0.696). In SGA infants, according to the twin charts, the incidence of abnormal umbilical artery Doppler was significantly more common than in SGA using the singleton chart (27.0% vs. 8.1%, p < 0.001). In conclusion, singleton charts misclassify a large number of twins as at risk of fetal growth restriction. The evidence suggests that the following twin-specific charts could reduce unnecessary medical interventions prenatally and postnatally.

Should Fetal Growth Charts Be References or Standards?

BACKGROUND

Fetal growth standards (prescriptive charts derived from low-risk pregnancies) are theoretically better tools to monitor fetal growth than conventional references. We examined how modifying chart inclusion criteria influenced the resulting curves.

METHODS

We summarized estimated fetal weight (EFW) distributions from a hospital's routine 32-week ultrasound in all nonanomalous singleton fetuses (reference) and in those without maternal-fetal conditions affecting fetal growth (standard). We calculated EFWs for the 3rd, 5th, 10th, and 50th percentiles, and the proportion of fetuses each chart classified as small for gestational age.

RESULTS

Of 2309 fetuses in our reference, 690 (30%) met the standard's inclusion criteria. There were no meaningful differences between the EFW distributions of the reference and standard curves (50th percentile: 1989 g reference vs. 1968 g standard; 10th percentile: 1711 g reference vs. 1710 g standard), or the proportion of small for gestational age fetuses (both 9.9%).

CONCLUSIONS

In our study, there was little practical difference between a fetal growth reference and standard for detecting small infants.

New charts for the assessment of body composition, according to air-displacement plethysmography, at birth and across the first 6 mo of life

BACKGROUND

Air-displacement plethysmography (ADP) is a good candidate for monitoring body composition in newborns and young infants, but reference centile curves are lacking that allow for assessment at birth and across the first 6 mo of life.

OBJECTIVE

Using pooled data from 4 studies, we aimed to produce new charts for assessment according to gestational age at birth (30 + 1 to 41 + 6 wk) and postnatal age at measurement (1-27 wk).

METHODS

The sample comprised 222 preterm infants born in the United States who were measured at birth; 1029 term infants born in Ireland who were measured at birth; and 149 term infants born in the United States and 57 term infants born in Italy who were measured at birth, 1 and 2 wk, and 1, 2, 3, 4, 5, and 6 mo of age. Infants whose birth weights were <3rd or >97th centile of the INTERGROWTH-21st standard were excluded, thereby ensuring that the charts depict body composition of infants whose birth weights did not indicate suboptimal fetal growth. Sex-specific centiles for fat mass (kg), fat-free mass (kg), and percentage body fat were estimated using the lambda-mu-sigma (LMS) method.

RESULTS

For each sex and measure (e.g., fat mass), the new charts comprised 2 panels. The first showed centiles according to gestational age, allowing term infants to be assessed at birth and preterm infants to be monitored until they reached term. The second showed centiles according to postnatal age, allowing all infants to be monitored to age 27 wk. The LMS values underlying the charts were presented, enabling researchers and clinicians to convert measurements to centiles and z scores.

CONCLUSIONS

The new charts provide a single tool for the assessment of body composition, according to ADP, in infants across the first 6 mo of life and will help enhance early-life nutritional management.

From population reference to national standard: new and improved birthweight charts

BACKGROUND

Antenatal detection of intrauterine growth restriction remains a major obstetrical challenge, with the majority of cases not detected before birth. In these infants with undetected intrauterine growth restriction, the diagnosis must be made after birth. Clinicians use birthweight charts to identify infants as small-for-gestational-age if their birthweights are below a predefined threshold for gestational age. The choice of birthweight chart strongly affects the classification of small-for-gestational-age infants and has an impact on both research findings and clinical practice. Despite extensive literature on pathological risk factors associated with small-for-gestational-age, controversy exists regarding the exclusion of affected infants from a reference population.

OBJECTIVE

This study aims to identify pathological risk factors for abnormal fetal growth, to quantify their effects, and to use these findings to calculate prescriptive birthweight charts for the Dutch population.

MATERIALS AND METHODS

We performed a retrospective cross-sectional study, using routinely collected data of 2,712,301 infants born in The Netherlands between 2000 and 2014. Risk factors for abnormal fetal growth were identified and categorized in 7 groups: multiple gestation, hypertensive disorders, diabetes, other pre-existing maternal medical conditions, maternal substance (ab)use, medical conditions related to the pregnancy, and congenital malformations. The effects of these risk factors on mean birthweight were assessed using linear regression. Prescriptive birthweight charts were derived from live-born singleton infants, born to ostensibly healthy mothers after uncomplicated pregnancies and spontaneous onset of labor. The Box-Cox-t distribution was used to model birthweight and to calculate sex-specific percentiles. The new charts were compared to various existing birthweight and fetal-weight charts.

RESULTS

We excluded 111,621 infants because of missing data on birthweight, gestational age or sex, stillbirth, or a gestational age not between 23 and 42 weeks. Of the 2,599,640 potentially eligible infants, 969,552 (37.3%) had 1 or more risk factors for abnormal fetal growth and were subsequently excluded. Large absolute differences were observed between the mean birthweights of infants with and without these risk factors, with different patterns for term and preterm infants. The final low-risk population consisted of 1,629,776 live-born singleton infants (50.9% male), from which sex-specific percentiles were calculated. Median and 10th percentiles closely approximated fetal-weight charts but consistently exceeded existing birthweight charts.

CONCLUSION

Excluding risk factors that cause lower birthweights results in prescriptive birthweight charts that are more akin to fetal-weight charts, enabling proper discrimination between normal and abnormal birthweight. This proof of concept can be applied to other populations.

Fetal biometric parameters: Reference charts for a non-selected risk population from Uberaba, Brazil

OBJECTIVE

To establish reference charts for fetal biometric parameters in a non-selected risk population from Uberaba, Southeast of Brazil.

METHODS

A retrospective cross-sectional study was performed among 5656 non-selected risk singleton pregnant women between 14 and 41 weeks of gestation. The ultrasound exams were performed during routine visits of second and third trimesters. Biparietal diameter (BPD) was measured at the level of the thalami and cavum septi pellucidi. Head circumference (HC) was calculated by the following formula: HC = 1.62*(BPD + occipital frontal diameter, OFD). Abdominal circumference (AC) was measured using the following formula: AC = (anteroposterior diameter + transverse abdominal diameter) × 1.57. Femur diaphysis length (FDL) was obtained in the longest axis of femur without including the distal femoral epiphysis. The estimated fetal weight (EFW) was obtained by the Hadlock formula. Polynomial regressions were performed to obtain the best-fit model for each fetal biometric parameter as the function of gestational age (GA).

RESULTS

The mean, standard deviations (SD), minimum and maximum of BPD (cm), HC (cm), AC (cm), FDL (cm) and EFW (g) were 6.9 ± 1.9 (2.3 - 10.5), 24.51 ± 6.61 (9.1 - 36.4), 22.8 ± 7.3 (7.5 - 41.1), 4.9 ± 1.6 (1.2 - 8.1) and 1365 ± 1019 (103 - 4777), respectively. Second-degree polynomial regressions between the evaluated parameters and GA resulted in the following formulas: BPD = -4.044 + 0.540 × GA - 0.0049 × GA² (R² = 0.97); HC= -15.420 + 2.024 GA - 0.0199 × GA² (R² = 0.98); AC = -9.579 + 1.329 × GA - 0.0055 × GA² (R² = 0.97); FDL = -3.778 + 0.416 × GA - 0.0035 × GA² (R² = 0.98) and EFW = 916 - 123 × GA + 4.70 × GA² (R² = 0.96); respectively.

CONCLUSION

Reference charts for the fetal biometric parameters in a non-selected risk population from Uberaba, Southeast of Brazil, were established.

Reference charts of fetal biometric parameters in 31,476 Brazilian singleton pregnancies

OBJECTIVES

The purpose of this study was to establish reference charts of fetal biometric parameters measured by 2-dimensional sonography in a large Brazilian population.

METHODS

A cross-sectional retrospective study was conducted including 31,476 low-risk singleton pregnancies between 18 and 38 weeks' gestation. The following fetal parameters were measured: biparietal diameter, head circumference, abdominal circumference, femur length, and estimated fetal weight. To assess the correlation between the fetal biometric parameters and gestational age, polynomial regression models were created, with adjustments made by the determination coefficient (R(2)).

RESULTS

The means ± SDs of the biparietal diameter, head circumference, abdominal circumference, femur length, and estimated fetal weight measurements at 18 and 38 weeks were 4.2 ± 2.34 and 9.1 ± 4.0 cm, 15.3 ± 7.56 and 32.3 ± 11.75 cm, 13.3 ± 10.42 and 33.4 ± 20.06 cm, 2.8 ± 2.17 and 7.2 ± 3.58 cm, and 256.34 ± 34.03 and 3169.55 ± 416.93 g, respectively. Strong correlations were observed between all fetal biometric parameters and gestational age, best represented by second-degree equations, with R(2) values of 0.95, 0.96, 0.95, 0.95, and 0.95 for biparietal diameter, head circumference, abdominal circumference, femur length, and estimated fetal weight.

CONCLUSIONS

Fetal biometric parameters were determined for a large Brazilian population, and they may serve as reference values in cases with a high risk of intrauterine growth disorders.

Reference charts for fetal corpus callosum length: a prospective cross-sectional study of 2950 fetuses

OBJECTIVES

The purpose of this study was to establish reference charts for fetal corpus callosum length in a convenience sample.

METHODS

A prospective cross-sectional study was conducted at the Artemisia Fetal-Maternal Medical Center between December 2008 and January 2012. Among 16,975 fetal biometric measurements between 19 weeks and 37 weeks 6 days' gestation, 3438 measurements of the corpus callosum (20.3%) were available. After excluding 488 measurements (14.2%), a total of 2950 fetuses (85.8%) were considered and analyzed only once. Parametric and nonparametric quantile regression models were used for the statistical analysis. To evaluate the robustness of the proposed reference charts with respect to various distributional assumptions on the sonographic measurements at hand, we compared the gestational age (GA)-specific reference curves produced by the statistical methods used.

RESULTS

The mean corpus callosum length was 26.18 mm (SD, 4.5 mm; 95% confidence interval, 26.01-26.34 mm). The linear regression equation expressing the length of the corpus callosum as a function of GA was length (mm) = -11.17 + 1.62 × GA. The correlation between the dimension and gestation was expressed by the coefficient r = 0.83. Normal mean lengths according the parametric and nonparametric methods were defined for each week of gestation.

CONCLUSIONS

This work provides new quantile-based reference charts for corpus callosum length measurements that may be useful for diagnosis of congenital corpus callosum anomalies in fetal life.