Methods
We conducted a secondary analysis of the prospective observational Nulliparous Pregnancy Outcomes Study: monitoring mothers-to-be (nuMoM2b), which has been described in detail elsewhere.16 Briefly, 10,038 pregnant participants were recruited from 8 U.S. clinical centers between 2010 and 2013. Recruitment was of singleton pregnancies with a 1sttrimester ultrasound and no prior deliveries beyond 20 weeks’ gestation. Study visits occurred at 6 0/7 – 13 6/7 weeks (visit 1), 16 0/7 – 21 6/7 weeks (visit 2), and at 22 0/7 – 29 6/7 weeks (visit 3) and included collection of clinical parameters, ultrasound, and detailed questionnaires. Outcomes were ascertained by medical record abstraction after birth. Ultrasounds generating EFWs using biparietal diameter, head circumference, abdominal circumference, and femur length took place at visits 2 and 3. EFW was calculated using the 4 fetal biometric parameters.17 The nuMoM2b study was approved by ethical review committees at participating institutions and was registered at clinicaltrials.gov (NCT01322529).
Our analysis included nuMoM2b participants who delivered at or beyond 24 weeks’ gestation with available delivery information. We excluded participants who underwent pregnancy termination, had missing key information, newborns not assigned a sex at birth, born after 41 weeks when birth weight percentile could not be calculated, or with implausible fetal weight measurements. From these, we selected a nested cohort of lower-risk participants to carry out our primary objective to derive a prescriptive, sex-specific fetal growth standard. For this nested cohort, we excluded those with pregnancies affected by several complications known to be associated with poor fetal growth, including preterm birth, preexisting hypertension, pre-gestational diabetes, suspected chromosomal or fetal anomalies, or stillbirth. To carry out our secondary objective to assess the sex-specific standard, we used the full cohort, including participants initially excluded for risk factors for abnormal growth so as to approximate an unselected population that would be more generalizable to clinical practice. Therefore, the analysis to address our second objective was carried out using the full eligible cohort and only excluded those missing key variables or whose pregnancies ended prior to 24 weeks.
Our primary endpoint was birth weight percentile. SGA was defined as birth weight <10th percentile, LGA was defined as birth weight >90th percentile, and appropriate for gestational age (AGA) was defined as birth weight 10th-90th percentile. Because the primary endpoints utilize birth weight, all weights <10th percentile are referred to as “SGA” in this analysis and “FGR” is only used when referring to nuMoM2b variables reflecting a prenatal diagnosis.
The Hadlock formula was used as the referent sex-neutral standard to calculate weight-for-age percentiles, which were applied to both EFWs and birth weights in order to maintain continuity between weights measured before and after delivery. Hereafter, the Hadlock standard is referred to as the “sex-neutral” standard. Birth weight percentiles were also assigned using the Olsen standard, a national sex-specific birth weight reference.8
To complete our primary objective, we used the lower-risk nested cohort to derive a prescriptive sex-specific fetal growth standard. To do this, we regressed EFWs and birth weights on fetal sex and weeks’ gestation using a longitudinal mixed-effects regression model to estimate an equation representing fetal growth. Two equation parameterization approaches were assessed; first, using fetal sex as an interaction term to determine whether a separate equation was needed for each sex; second, using sex as an intercept only, which uses the same equation for both sexes but from different starting points. Our equation was modeled after the Hadlock approach because of its simplicity and ease of clinical use. The Hadlock method calculates a z score for the difference between the observed and expected weight and transforms the z-score into a percentile.11 The standard deviation for estimated fetal weight was calculated from an approach mirroring that from the original Hadlock publication. For each week of gestation, the residual standard deviation as a percent of the predicted weight was calculated. The average of percent residual standard deviation across weeks is reported as the standard deviation of fetal weights. Once the sex-specific, longitudinal equation was finalized, percentiles were calculated for birth weights using the new sex-specific standard, the sex-neutral standard, and the Olsen birth weight standard, comparing the proportion of SGA and LGA newborns by fetal sex using each. Rather than assume that male and female fetuses should automatically have the same proportions of SGA or LGA, we used a validated sex-specific birth weight standard (Olsen) as a reference for the expected rates of SGA and LGA for each sex.
Our secondary objective was to compare metrics of clinical outcomes and management according to growth status using sex-neutral versus sex-specific growth standards in an unselected cohort. To do this, we used the full eligible cohort. We then compared interventions and perinatal outcomes between two size classifications: newborns designated as AGA by both sex-neutral and sex-specific standards, and newborns whose growth status was reclassified by the sex-specific standard. This comparison was performed separately for male and female fetuses as well as for SGA and LGA.
Two types of clinical measures were used to assess the sex-specific growth standard. First, we assessed a composite of perinatal morbidity, which was defined as stillbirth occurring ≥ 24 weeks, need for mechanical ventilation, neonatal death before discharge, NICU stay > 48 hours, confirmed sepsis, respiratory distress syndrome, seizures, necrotizing enterocolitis (NEC), and intraventricular hemorrhage (IVH). Second, we selected measures that were specific to SGA and LGA to assess the clinical relevance of the sex-specific standard. Measures specific to SGA included admission to labor and delivery for FGR, delivery for FGR, clinical suspicion of FGR before delivery, scheduled labor induction or cesarean without labor before 39 weeks’ gestational age, and cesarean delivery for non-reassuring fetal status. Clinical outcomes specific to LGA included cesarean delivery for arrest of dilation, cesarean for arrest of descent, shoulder dystocia, and brachial plexus injury. This analysis was not an effort at internal validation, but rather an exploratory assessment of whether reclassification from normal to abnormal orvice versa by the sex-specific standard was inappropriate.
Comparisons of SGA and LGA overall, and comparisons of clinical outcomes and management by SGA and LGA were tested with a chi-square test.
Statistical analysis was performed using SAS software, Version 9.4 of the SAS System for Windows. Copyright © 2006 SAS Institute Inc. Cary, NC, USA. Graphics were created using GraphPad Prism version 9.1.2 for Windows, GraphPad Software, La Jolla California USA.