Sex-specific antenatal reference growth charts for uncomplicated singleton pregnancies at 15-40 weeks of gestation

Female fetuses, on average, weigh less than male fetuses at all gestational ages. The purpose of this study was to compare female and male fetuses in terms of intrauterine ultrasound growth measurements and to develop gestational-age-related charts based on a computerized perinatal database.

This was a retrospective study of unselected women in the second and third trimesters of pregnancy, who had a normal scan at 10-14 weeks. Data analysis was performed using measurements obtained from a mixed-race population of 4234 women, who underwent 5198 ultrasound examinations. The scans were performed by four trained sonographers, according to a standardized protocol. Routine measurements included biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL). The main end-points were sex- and race-specific differences in fetal biometry, which were also used to estimate fetal weight.

The base-line demographic characteristics and risk factors were comparable in female and male fetuses. Significant differences in fetal BPD, HC, AC and estimated fetal weight, but not FL, were seen between male and female fetuses. Centile charts for each of these variables were constructed for both male and female fetuses.

This study suggests that small but consistent sex-related differences in prenatal BPD, HC and AC measurements are established by as early as 15 weeks of gestation. The use of sex-specific nomograms may improve the prenatal assessment of fetal growth as well as the diagnosis of structural abnormalities.