Fetal nuchal translucency: ultrasound screening for fetal trisomy in the first trimester of pregnancy.

OBJECTIVE

To investigate a new method of screening for fetal trisomies on the basis of maternal age and fetal nuchal translucency thickness at 10 to 13 weeks of gestation.

DESIGN

A prospective screening study.

SETTING

Tertiary referral centre.

SUBJECTS: One thousand two hundred and seventy-three women with singleton pregnancies undergoing first trimester fetal karyotyping because of advanced maternal age, parental anxiety, or family history of a chromosomal abnormality in the absence of balanced parental translocation.

METHODS

Estimates of maternal age-related risks for fetal trisomies 21, 18 and 13 at this gestation were used to derive the expected incidence of these trisomies in fetuses with nuchal translucency < 3 mm, 3 mm and > 3 mm, respectively, and the ratio of observed to expected number of cases was calculated.

RESULTS

The nuchal translucency was > or = 3 mm in 86% of the trisomic and in 4.5% of the chromosomally normal fetuses. The observed number of trisomies in the 1185 cases with nuchal translucency < 3 mm was approximately five times less than the number expected on the basis of maternal age. In the groups with translucency of 3 mm (n = 52) and > 3 mm (n = 36), the observed numbers of trisomies were approximately five times and 24 times higher than the respective numbers expected on the basis of maternal age.

CONCLUSION

The risk of fetal trisomy can be derived by combining maternal age and fetal nuchal translucency thickness at 10 to 13 weeks of gestation. It is predicted that for a false positive rate of 5%, the sensitivity of the new method of screening would be at least 85%, which compares favourably with the respective 20 to 30% and 50 to 60% of screening based on maternal age alone or the combination of maternal age with maternal serum biochemistry.