Fetal cephalometry by ultrasound as a screening procedure for the prenatal detection of Down's syndrome.

Infants and children with Down's syndrome have a cephalic index (ratio of biparietal to occipitofrontal diameter) higher than that in normal children. To determine whether this difference is present and detectable by ultrasound measurement of the second-trimester fetal head, we calculated the cephalic indices for 308 normal fetuses and eight fetuses with a 47, +21 karyotype. The mean cephalic index in the fetuses with Down's syndrome (0.829, SD 0.033) was indistinguishable from that in the normal fetuses (0.825, SD 0.042). These data suggest that the documented difference in mean cephalic index between liveborn children with Down's syndrome and normal control children is not detectable in the mid-gestation fetus and that ultrasound cephalometry alone is unlikely to discriminate reliably between normal and affected fetuses.