Study Finds Racial, Ethnic Differences in Fetal Growth

Current standards for ultrasound evaluation of fetal growth may lead to misclassification of up to 15 percent of fetuses of minority mothers as being too small, according to a study by researchers at the National Institutes of Health (NIH) and other institutions.

Fetal growth restriction is a medical term used to describe fetuses that do not keep up with growth milestones appropriate to their stage of development. Growth restriction is a sign of an underlying health problem, often resulting from the fetus not receiving enough nutrients or oxygen in the uterus.

According to the latest study, however, many fetuses of minority mothers may be developing normally, but because of hereditary and environmental factors, they are smaller than their white counterparts. The findings could mean that many otherwise healthy pregnant women are subjected to tests and procedures that they don’t need.

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The study, based on serial scans of more than 1,700 pregnancies, was published online in the American Journal of Obstetrics and Gynecology.

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Currently, many practitioners rely on older reference charts for estimating whether a fetus is growing normally. During a sonogram, measurements are taken of the fetus’ head circumference, abdominal circumference and length of the femur (thigh bone). These measurements are used in a formula to estimate the weight of the fetus, which is then compared to a series of estimated weights on a chart of fetal weight stratified by gestational age. The growth chart weights were derived from a study by Frank Hadlock and his colleagues, who compiled ultrasound measurements of 139 pregnancies of predominantly middle-class white women during the 1980s.

Today, the current study authors wrote, new mothers tend to be older, heavier, and more likely to be non-white than when the Hadlock entries were compiled. The researchers sought to compile standards that more accurately reflect the best fetal growth during healthy pregnancies among the most common racial and ethnic groups in the United States. {snip}

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The researchers found significant differences among fetuses in the different groups after the 20th week of pregnancy. For example, by the 39th week, fetuses of white mothers were the largest at 4402 grams, followed by fetuses of Hispanic mothers (4226 grams). Fetuses of black mothers were the smallest, at 4053 grams.

The researchers then scored the estimated weights of the fetuses of minority mothers based on the weights they had compiled for fetuses of white mothers. They found that, depending on the group and the week of pregnancy, 5 to 15 percent of the fetuses of minority mothers scored below the 5th percentile when compared to fetuses of white mothers. For example, at 35 weeks of pregnancy, 14 percent of fetuses of black mothers and 15 percent of fetuses of Asian mothers would have been classified as below the 5th percentile, based upon the white standard.

The authors added that future studies to determine fetal growth standards should attempt to take racial and ethnic differences into account. {snip}