2 Methods
2. Study Setting and Population
Our study was a secondary analysis of the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be (nuMoM2b), a large prospective observational cohort study designed to assess contributors to adverse pregnancy outcomes. Detailed nuMoM2b protocols were previously published and are briefly summarized here.(13) Participants in the parent study were recruited at 8 geographically diverse U.S. sites from 2010-2013 and were included if they had a singleton pregnancy between 6 weeks 0 days and 13 weeks days’ gestation and no prior pregnancies lasting 20 weeks or more. Potential participants were excluded for age <13 years, 3 or more prior miscarriages, suspected fetal malformation at the time of enrollment, known fetal aneuploidy, conception using a donor oocyte, multifetal reduction, plan for pregnancy termination, or participation in an intervention study to influence pregnancy outcomes. Participants had 4 study visits: during approximately the first, second, and early third trimesters of pregnancy, as well as one after delivery. For this secondary analysis, we included all participants with a live birth at ≥24 weeks with available maternal single nucleotide polymorphism (SNP) array data, derived from unselected maternal blood collection that was part of the protocol for all parent study participants. Participants were excluded if they did not complete any of the 3 research ultrasounds or were missing key variables including fetal sex and BW.
2.2 Outcomes
The primary outcome of this study was association of one’s genetic risk score for infant BW with race. Race was divided into self-identified race and genetic ancestry. The secondary outcome of this study was to assess the relationship and overlap between self-identified race and genetic ancestry.
Race designations were self-identified from among the following: White, Black/African American, Asian, Native Hawaiian/Other Pacific Islander, American Indian/Alaskan Native, Multiracial, and Unknown/not reported. Genetic ancestry was ascertained using Peddy, a software package that uses an individual’s DNA to predict the predominant continental ancestry(14) with the following categorical outputs: AFR, African; AMR, American (Indigenous); EAS, East Asian; EUR, European; SAS, South Asian; UNK, unknown. We assessed the distribution of predicted genetic ancestry within self-identified racial groups.
Maternal DNA was isolated from blood collected at visit 1. Genotyping was performed using a commercially available kit (Infinium Multi-Ethnic Global D2 Bead Chip; Illumina), from which SNP arrays were conducted based on the Genome Reference Consortium human build 38 (CRCh38).(15) 86 BW-associated SNPs that were identified using GRCh37(7) were mapped to GRCH38 for compatibility, yielding 73 SNPs. Maternal SNP arrays were used to compute the GRSBW for each maternal participant using the weighted sum of BW-associated variants present in each person, such that the score represents the cumulative effect size without traditional units, expressed as GRS = (V11) + (V21) + …(V7373) , whereV1 is variant 1 and β1 is the effect size for variant 1.
2.3 Statistical Analysis
A log-linear model was used to test the association between maternal GRSBW with infant BW, controlling for fetal sex and gestational age at birth. To assess generalizability of the GRSBW across self-identified racial groups, the association between GRSBW and BW was assessed for each self-identified racial subgroup using stratified log-linear models. This same approach was repeated across groups defined by genetic continental ancestry. Finally, self-identified race and genetic ancestry were included as predictor variables in separate log-linear models to test whether they remained independently associated with BW after controlling for GRSBW, infant sex, and gestational age.