Authors' Reply: The assessment of combined first trimester screening in women of advanced maternal age in an Asian cohort.

Abstract

Dear Sir, We thank Lai and Yeo(1) for their comments regarding our study on the performance of first trimester screening (FTS) in women of advanced maternal age in a Southeast Asian population. We are delighted to hear that the performance of FTS at KK Women’s and Children’s Hospital is similar to that at National University Hospital and Singapore General Hospital, since it suggests that high standards are being maintained throughout Singapore. Our data was not selected from a high-risk population, but was derived from consecutive cases presenting for FTS at the two clinics. The prevalence rate for the trisomies within this unselected population is very similar to that seen in another study of over 10,000 all-risk Caucasian women carried out in Western Australia.(2) FTS is offered to all women in Singapore, but not taken up by all; intuitively, it seems logical that as women get older, they will be more likely to wish to screen for aneuploidy following counselling from their doctor. Therefore, the percentage of older women undergoing FTS is likely to be higher than the total percentage of women in that age group giving birth at any particular centre, causing a self-selection bias in the screening population. We stated in our study that the pregnancy outcome was known either by the karyotype data or birth outcome; we later went on to explain that the birth outcome data was a limitation of the study, since we are relying on a feedback loop from the delivering hospitals to notify us if there is a case of aneuploidy discovered at birth. We acknowledge that there may be some shortcomings in this system, and we will be keen to enlist the help of the National Birth Defects Registry as suggested for future studies. Yours sincerely,