First-trimester fetal nuchal translucency thickness and risk for trisomies.

OBJECTIVE: To define the relation between fetal nuchal translucency thickness at 10-13 weeks' gestation and the risk for fetal trisomies and pregnancy outcome.
METHODS: Five hundred sixty fetuses with nuchal translucency thickness of 3-9 mm at 10-13 weeks' gestation were karyotyped. The ratio of the observed number of fetal trisomies to that expected on the basis of maternal age was calculated.
RESULTS: The incidence of trisomies 21, 18, or 13 was 18% (102 of 560 cases) and was significantly associated with both maternal age (r = 0.97) and fetal nuchal translucency thickness (r = 0.75). In 383 fetuses with nuchal translucency of 3 mm, the observed number of fetal trisomies was 23, in contrast to the frequency of 6.0 expected on the basis of maternal age. In 177 fetuses with nuchal translucency of 4 mm or more, 79 cases were observed, compared with 2.7 expected on the basis of maternal age. In fetuses with nuchal translucency of 4 mm or more and normal karyotype, there was a high association with other defects and the prognosis was often poor, whereas the translucency resolved for those with 3 mm and the pregnancy outcome was usually normal.
CONCLUSION: At 10-13 weeks' gestation, fetal nuchal translucency of 3 mm is associated with a fourfold increase, and translucency of greater than 3 mm with a 29-fold increase, in the maternal age-related risk for trisomies 21, 18, and 13. Fetal nuchal translucency of 4 mm or more is associated with poor pregnancy outcome even when the fetal karyotype is normal.