FIRST-TRIMESTER FETAL NUCHAL TRANSLUCENCY THICKNESS AND RISK FOR TRISOMIES

Objective: To define the relation between fetal nuchal translucency th ickness at 10-13 weeks' gestation and the risk for fetal trisomies and pregnancy outcome. Methods: Five hundred sixty fetuses with nuchal tr anslucency thickness of 3-9 mm at 10-13 weeks' gestation were karyotyp ed. The ratio of the observed number of fetal trisomies to that expect ed 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 signifi cantly associated with both maternal age (r = 0.97) and fetal nuchal t ranslucency thickness (r = 0.75). In 383 fetuses with nuchal transluce ncy of 3 mm, the observed number of fetal trisomies was 23, in contras t to the frequency of 6.0 expected on the basis of maternal age. In 17 7 fetuses with nuchal translucency of 4 mm or more, 79 cases were obse rved, compared with 2.7 expected on the basis of maternal age. In fetu ses with nuchal translucency of 4 mm or more and normal karyotype, the re was a high association with other defects and the prognosis was oft en poor, whereas the translucency resolved for those with 3 mm and the pregnancy outcome was usually normal. Conclusion: At 10-13 weeks' ges tation, fetal nuchal translucency of 3 mm is associated with a fourfol d increase, and translucency of greater than 3 mm with a 29-fold incre ase, in the maternal age-related risk for trisomies 21, 18, and 13. Fe tal nuchal translucency of 4 mm or more is associated with poor pregna ncy outcome even when the fetal karyotype is normal.