FETAL NUCHAL TRANSLUCENCY - ULTRASOUND SCREENING FOR FETAL TRISOMY INTHE 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 an d seventy-three women with singleton pregnancies undergoing first trim ester fetal karyotyping because of advanced maternal age, parental anx iety, or family history of a chromosomal abnormality in the absence of balanced parental translocation. Methods Estimates of maternal age-re lated risks for fetal trisomies 21, 18 and 13 at this gestation were u sed to derive the expected incidence of these trisomies in fetuses wit h nuchal translucency < 3 mm, 3 mm and > 3 mm, respectively, and the r atio of observed to expected number of cases was calculated. Results T he nuchal translucency was greater than or equal to 3 mm in 86 % of th e trisomic and in 4.5 % of the chromosomally normal fetuses. The obser ved 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 approxim ately five times and 24 times higher than the respective numbers expec ted on the basis of maternal age. Conclusion The risk of fetal trisomy can be derived by combining maternal age and fetal nuchal translucenc y 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 scre ening would be at least 85 %, which compares favourably with the respe ctive 20 to 30 % and 50 to 60 % of screening based on maternal age alo ne or the combination of maternal age with maternal serum biochemistry .