INTRODUCTIONFirst trimester screening (FTS) is a validated screening modality for major chromosomal aneuploidies (i.e. trisomies 21, 18 and 13).(1-3) Using a combination of maternal age, fetal nuchal translucency thickness, maternal levels of serum free β-human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein A (PAPP-A), FTS has been shown to achieve a detection rate of 84%-90%, with a false positive rate of 5%.(4-6) Although FTS has been adopted in many countries as the first-line screening for trisomy 21 (i.e. Down syndrome), its effectiveness in women of different ages has not been adequately assessed. A recent observational study reported that FTS is less effective in women aged < 35 years and that a greater number of invasive procedures needs to be performed in this group of women to diagnose a case of trisomy 21.(7) This finding is contrary to reports on other Caucasian populations. (8)(9)(10) If the performance of FTS varies in women of different ages, it is clinically pertinent that these variations are understood. This is especially since the proportion of pregnant women of advanced maternal age, defined as age 35 years and above in most countries, has increased in the past decade. It is unclear whether the widely published detection rates in the literature can be universally applied to women of all ages, especially those of advanced maternal age, as they appear to have a higher risk for fetal trisomy 21.There has been a growing body of evidence regarding the high negative predictive value of noninvasive prenatal testing (NIPT) using cell-free DNA (cfDNA) and its potential to reduce iatrogenic fetal loss. (11)(12)(13)(14) Several countries have already started to recommend the use of NIPT using cfDNA for contingent screening in women who are at a higher risk of a fetus with trisomy 21, including women of advanced maternal age. (15)(16)(17)(18) There is, however, a lack of information regarding the potential incremental costs that follow contingent screening, especially in Singapore.The objectives of the present study were to assess the performance of FTS in an ethnically mixed population for the detection of fetal trisomy 21, and to compare the effectiveness of FTS in women of advanced maternal age (i.e. ≥ 35 years) with that of FTS in women aged < 35 years. We also aimed to determine the incremental cost of detecting one case of trisomy 21 if NIPT was offered to all women who had intermediate-or high-risk scores following FTS.