First-trimester combined screening for trisomy 21 in a predominantly Chinese population.

OBJECTIVE To examine the effectiveness of first-trimester fetal trisomy 21 screening using a combination of maternal age, nuchal translucency thickness (NT) and maternal serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) levels in a predominantly Chinese population in Hong Kong. METHODS This was a prospective study over a 1.5-year period of 2990 women who underwent combined screening for trisomy 21 between 11+0 and 13+6 weeks of gestation in a university fetal medicine unit. NT was measured according to the criteria set by The Fetal Medicine Foundation (FMF), maternal serum free beta-hCG and PAPP-A levels were measured, and the risk of trisomy 21 was calculated using The FMF's algorithm. Fetal karyotyping was advised when the risk was 1 : 300 or above. All subjects were followed up for pregnancy and fetal outcome. RESULTS Of the 2990 women who underwent the screening program, 99% were Chinese. There were 57 twin pregnancies, giving a total of 3047 fetuses. Thirty-one percent of the women were 35 years old or above. One hundred and eighty-five (6.1%) fetuses were screen-positive; this included 14 cases of trisomy 21 and 17 cases of other chromosomal abnormalities. The positive predictive value was 16.7%. Among the 2862 screen-negative fetuses, only 18 (0.6%) cases had an unknown fetal outcome. There were no cases in which trisomy 21 was missed and the infant was liveborn. CONCLUSION First-trimester combined screening for fetal trisomy 21 is highly effective among Chinese subjects.