Sex bias in ultrasound measures of gestational age: assessment by sex ratio in post-term births.

Background: Estimation of fetal age by ultrasound assumes identical biometries for both sexes at identical gestational ages. However, late in the first trimester male fetuses become larger overall than female fetuses, which may introduce a sex bias with clinical consequences. A recent study showed that using due-date estimates based on biparietal diameter from the second trimester increased the post-term male-to-female ratio and the risk of stillbirth among female fetuses born at 43 gestational weeks.