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.