INTRODUCTIONFirst trimester screening (FTS) is a validated screening tool that has been shown to achieve detection rates of 84%-90% for trisomies 21, 18 and 13. However, its effectiveness for different maternal ages has not been assessed. The present study aimed to assess the performance of FTS in an Asian population, and to compare its effectiveness in older (≥ 35 years) and younger (< 35 years) women. The potential use of noninvasive prenatal test (NIPT) as a contingent screening test is also examined.

METHODSData on cases of FTS performed on singleton pregnancies over a six-year period was collated from two Singapore maternal centres, National University Hospital and Singapore General Hospital. Cases that had a 1:250 risk of trisomy were considered to be screen-positive. Pregnancy outcomes were obtained from birth records or karyotype test results.

RESULTSFrom 10,289 FTS cases, we obtained a sensitivity of 87.8%, a specificity of 97.6%, a false positive rate of 2.4% and a false negative rate of 0.06% for the detection of aneuploidy. The overall detection rate for trisomy 21 was 86.5%-85.7% for older women and 87.5% for younger women. The mean number of invasive tests required per case of trisomy 21 was 9.3 in younger women, 8.6 in older women and 13.5 in women with intermediate risk (1:250-1,000).

CONCLUSIONWhile the performance of FTS was similar in younger and older women, more invasive procedures were required to diagnose trisomy 21 in women with intermediate risk. It may be advantageous to offer contingent NIPT to this group of women to reduce the risk of iatrogenic fetal loss.