Down syndrome screening in Singapore--the effectiveness of a second trimester serum screening policy modelled on 29,360 pregnancies in KK Women's and Children's Hospital

Abstract

Aim of study: To assess the effectiveness of a proposed second trimester Down syndrome serum screening policy in Singapore.

Method: Auditing the effectiveness of an age-only policy and comparing it against a serum screening policy modelled on the same maternal population of KK Women's and Children's Hospital in 1994 and 1995.

Results: KK Women's and Children's Hospital's (KKH) maternal age distribution is similar to the national age distribution of mothers. Sixteen percent (16.7%) of mothers in KKH, in 1994 and 1995, were 35 years or older at delivery. Based on our hospital birth defect registry, 66% (35/53) of Down Syndrome pregnancies occurred in mothers who were 35 years or older at delivery and 43% (23/53) in the oldest 6.5% of mothers (38 years or older at delivery). Using various models on KKH's population structure to estimate the expected number of Down Syndrome livebirths expected, 52%-55% and 34%-36% of Down Syndrome livebirths were expected to occur in the oldest 16.7% and 6.5% of mothers respectively. These simulated figures are much lower than the figures from the data and needs further study, assuming that the Western Down Syndrome risk model to be applicable to our population. The overall uptake of amniocentesis irrespective of gestational age at booking was 28%. In mothers who were 35 years or older at delivery and booked before 22 weeks gestation, the uptake rate of amniocentesis was 49%. There was a substantial difference in the uptake rate when the counselling was done by trained counsellers compared to those who were not.

Conclusion: We would expect that for a fixed amniocentesis rate of 6.5% and 16.7%, serum screening would be able to detect 71% and 85% respectively of the Down syndrome pregnancies. This is more efficient than figures published from Western populations as our patients are older.