Normal fetal growth varies by race

Every expecting mom knows that one of the parameters obstetricians have for measuring the health of an unborn baby is, "Is s/he well grown?" Obstetricians can measure a mom-to-be's "fundal height" by taking a ruler and measuring from the pubic bone to the top of the uterus. If this measurement is off, or if there is some other reason, the obstetrician can kick it up a notch and use an ultrasound measurement of the fetus's size. Lots of moms have experienced these ultrasounds. The ultrastenographer carefully measures the fetus's bone lengths, head, and belly to come up with a computer-generated number called the Estimated Fetal Weight or EFW. Doctors will take the EFW and compare it to other fetuses of the same gestational age. Just like pediatricians do for kids, obstetricians will compare the individual EFW to the normal range along a growth curve. If the EFW is less than the 5th percentile of the normal curve, the fetus meets criteria for "growth restriction."

As you might expect, these growth curves are very helpful in deciding what to call "normal growth" and "abnormal growth." However, there are certain times when you have a small momma and poppa and wonder to yourself, "Does this low EFW really represent something worrisome?" The original growth curves were created using low risk pregnancies without racial and ethnic diversity. With increased diversity in the United States, are we doing a disservice to these little ones by comparing them to the wrong growth curve for them?

A large collaborative group from the National Institute of Child Health and Human Development, led by Germaine Buck Louis, PhD, asked just this question of a large group of ethnically diverse expectant mothers. The researchers wanted to know if a normal growth curve differs for four generalized racial/ethnic groupings: non-Hispanic white, non-Hispanic black, Hispanic, and Asian. They enrolled 2,334 healthy women with low risk singleton pregnancies at 12 different hospitals from 2009-2013. The break-down in race was: 26% white, 26% black, 28% Hispanic, and 20% Asian. 

The researchers followed a stringent ultrasound schedule. They confirmed the gestational age for every pregnancy using ultrasounds between 8 and 13.9 weeks. They then assigned the expectant mothers to receive ultrasounds throughout their pregnancies, staggering the ultrasounds so the authors would have data on every week without making every mother come in for an ultrasound every week. A very clever design indeed!

The authors wanted to capture a "normal" or healthy growth curve, so they excluded anyone who developed complications over the course of their pregnancy. Some women were also not able to follow the entire ultrasound protocol, so the authors didn't include these women in the final analysis either. In the end, one quarter of the women had to be excluded from the analysis for these reasons, leaving 1,737 women whose growth measurements were used to estimate what is normal. 

The results showed just what the authors hypothesized. There were significant differences between the four broad racial groups in what comprised a normal growth curve. Below is a very basic graph of the data for 39 weeks, showing that fetuses born to white mothers had the highest weights, followed by Hispanic, then Asian, then black mothers.

One of the most amazing findings of this important study was that differences in growth emerged very early. EFW was significantly different by the second trimester and individual differences in the various measurements emerged even earlier. 

The results are obvious: growth curves definitely differ depending on the mother's race/ethnicity. But- does this make any difference in clinical practice? In fact it does! Based on the findings in this study, 15% of non-white fetuses were incorrectly classified as growth restricted (EFW less than the 5th percentile) when using the white growth curve. More research is definitely needed. For instance, these four racial groups are quite diverse themselves. For instance, the "Asian" group includes East Asian women (such as Chinese women), South Asian women (such as Indian women), Pacific Islanders, and Southeast Asian women (such as Thai women). Furthermore, the groups were only based on the mother's race/ethnicity and not the father's. One thing is certain: the time has come for better delineation of normal fetal growth curves, using more than just gestational age!