Fetal Sex and Intrauterine Growth Patterns

OBJECTIVES: To analyze the effect of fetal sex on intrauterine growth patterns during the second and third trimesters. METHODS: We conducted a cross-sectional study of women with uncomplicated singleton pregnancies who underwent sonographic fetal weight estimation during the second and third trimesters in a single tertiary center. The effect of fetal sex on intrauterine growth patterns was analyzed for each of the routine fetal biometric indices (biparietal diameter, head circumference, occipitofrontal diameter, abdominal circumference, and femur length) and their ratios. Sex-specific regression models were generated for these indices and their ratios as a function of gestational age. Sex-specific growth curves were generated from these models for each of the biometric indices and their ratios for gestational weeks 15 to 42. RESULTS: Overall, 12,132 sonographic fetal weight estimations were included in the study. Fetal sex had an independent effect on the relationship between each of the biometric indices and their ratios and gestational age. These effects were most pronounced for biparietal diameter (male/female ratio, 1.021) and the head circumference/femur length and biparietal diameter/femur length ratios (male/female ratios, 1.014 and 1.016, respectively). For the head measurements, these sex-related differences were observed as soon as the early second trimester, whereas for abdominal circumference, the differences were most notable during the late second and late third trimesters. CONCLUSIONS: Female fetuses grow considerably slower than male fetuses, and these differences are observed from early gestation. However, the female fetus is not merely a smaller version of the male fetus, but, rather, there is a sex-specific growth pattern for each of the individual fetal biometric indices. These findings provide support for the use of sex-specific sonographic models for fetal weight estimation as well as the use of sex-specific reference growth charts.