OP25.05: First trimester combined screening for trisomy 21 in a predominantly Chinese population

Abstract

To examine the effectiveness of first-trimester fetal Trisomy 21 (T21) screening using a combination of maternal age, nuchal translucency (NT), maternal serum free beta-hCG and PAPP-A levels in a predominantly Chinese population in Hong Kong. A prospective study performed in a university fetal medicine unit over a three and a half-year period. Consecutive women who underwent the combined screening for T21 between 11 and 13 + 6 weeks of gestation were recruited. NT was measured according to the criteria set by the Fetal Medicine Foundation (FMF). Maternal serum biochemistry was measured by Kryptor analyzer. Risk of T21 was calculated using FMF's algorithm. Fetal karyotyping was advised when the risk was 1 : 300 or above for T21, or 1 : 150 for T13/18. All subjects were followed up for pregnancy and fetal outcome. Among the 9333 women who underwent the screening program, 99% were Chinese and 173 women had a twin pregnancy, with a total of 9506 fetuses. 27.1% of the women were at or above 35 years old. 558 fetuses were screened positive (5.98%) for T21 (with or without increased risk for T13/18). Another 18 cases were positive for T13/18 only, making a total of 576 (6.17%). There were a total of 26 cases of T21, of which two were screened negative (detection rate of 92.3%). There were five cases of T13, 12 cases of T18, 10 cases of monosomy X, and six cases of other chromosomal abnormalities; all were screened positive. The overall positive predictive value for chromosomal abnormality was 1 in 9.9. First-trimester combined screening for fetal T21 is highly effective among Chinese subjects.