Racial/ethnic standards for fetal growth: the NICHD Fetal Growth Studies

Abstract

Objective Fetal growth is associated with long-term health yet no appropriate standards exist for the early identification of under- or over-grown fetuses. We sought to develop contemporary fetal growth standards for four self-identified U.S. racial/ethnic groups. Study Design We recruited for prospective follow-up 2,334 healthy women with low-risk, singleton pregnancies from 12 community and perinatal centers between July 2009 and January 2013. The cohort comprised: 614 (26%) non-Hispanic Whites, 611 (26%) non-Hispanic Blacks, 649 (28%) Hispanics, and 460 (20%) Asians. Women were screened at 8w0d to 13w6d for maternal health status associated with presumably normal fetal growth (aged 18–40 years; body mass index 19.0–29.9 kg/m2; healthy lifestyles and living conditions; low-risk medical and obstetrical history); 92% of recruited women completed the protocol. Women were randomized among four ultrasonology schedules for longitudinal fetal measurement using the Voluson E8 GE Healthcare. In-person interviews and anthropometric assessments were conducted at each visit; medical records were abstracted. The fetuses of 1,737 (74%) women continued to be low-risk (uncomplicated pregnancy, absent anomalies) at birth, and their measurements were included in the standards. Racial/ethnic-specific fetal growth curves were estimated using linear mixed models with cubic splines. Estimated fetal weight and biometric parameter percentiles (5th, 50th, 95th) were determined for each gestational week and comparisons made by race/ethnicity, with and without adjustment for maternal and socio-demographic factors. Results Estimated fetal weight differed significantly by race/ethnicity after 20 weeks. Specifically at 39 weeks, the 5th, 50th, and 95th percentiles were 2790, 3505, and 4402 grams for White, 2633, 3336, and 4226 grams for Hispanic, 2621, 3270, and 4078 grams for Asian, and 2622, 3260, and 4053 grams for Black women (adjusted global p<0.001). For individual parameters, racial/ethnic differences by order of detection were: humerus and femur lengths (10 weeks), abdominal circumference (16 weeks), head circumference (21 weeks), and biparietal diameter (27 weeks). The study-derived standard based solely on the White group erroneously classifies as much as 15% of non-White fetuses as growth-restricted (estimated fetal weight < 5th percentile). Conclusions Significant differences in fetal growth were found among the four groups. Racial/ethnic-specific standards improve the precision in evaluating fetal growth.

A new customized fetal growth standard for African American women: the PRB/NICHD Detroit study

“…Given the ethnic differences in EFW reported in the NICHD study 21 and in birthweight data reported herein, combined with the limited number of Caucasian women in our study population, we decided to focus on developing a customized fetal growth standard for African American women. Non-customized centiles (5 th , 10 th , 50 th , 90 th , and 95 th ) of fetal biometric parameters, EFW, and indices of proportionality for all 4,001 African American women (regardless of clinical outcome) are shown in Figure S2.…”

“…The resulting population-level centiles (i.e. non-customized and representing the entire study population) were superimposed on the raw data for visualization purposes and compared to other non-customized standards, such as the NICHD African American standard 21 and the WHO standard non-customized by fetal sex.…”

“…Ethnic differences in fetal biometric parameters were also recently assessed by other investigators for women with a low-risk pregnancy 21 . The difference in mean birthweight between African American and Caucasian women at term in the study herein was about one-half (133g) compared to that reported in the NICHD study (246g) 21 .…”

“…One issue is whether the same standard, referred to as “population-based,” should be used for all fetuses 16 , or whether the standard should be customized for physiologic and constitutional factors known to affect neonatal size at birth 17-19 as well as EFW 20, 21 .…”

“…For example, the INTERGROWTH-21 st study 16, 38-40 proposed a one-size-fits-all standard derived from a multi-ethnic population. By contrast, the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) fetal growth studies 21 reported standards specific to four different ethnic-racial groups (non-Hispanic White, Hispanic, African American, and Asian) 21 , yet customization by factors other than race was not provided. Recently, a study sponsored by the World Health Organization (WHO) 20, 41 proposed a multi-ethnic growth standard customized only by fetal sex, despite the observation that other factors (e.g.…”