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

Abstract

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.