Nuchal translucency in the first trimester.

All fetuses reported as having nuchal (nape of the neck) thickening or translucency detected by a first trimester ultrasound at 2 centres were reviewed to determine the incidence of aneuploidy. Twenty nine pregnancies were registered, amongst which 12 (41%) had an abnormal karyotype, the most common aneuploidy being trisomy 21 (5 fetuses). The likelihood of aneuploidy increased with increasing thickness of the translucency. Where the karyotype was found to be normal, there was complete resolution of this ultrasound appearance by the second trimester in 15 of the 16 ongoing pregnancies: 9 have delivered normal neonates and 6 pregnancies are continuing. The other fetus developed evidence of a skeletal dysplasia by the second trimester.