Abstract
Objective To analyse the distribution and determinants of the first trimester screening risk
for the detection of trisomy 21 in the population in and around Chennai, a south Indian
metropolitan city.
Methods A cross-sectional analysis of 27,647 singleton pregnancies that underwent the first
trimester combined screening test (FTS) was carried out. For the screen positive cases,
karyotype reports or postnatal phenotype outcome were available. The distribution
of the various components of risk assessment in the screen positive cases and screen
negative cases formed the main outcome measures.
Results Of the 27,647 cases, 4.6% (1270) of cases had unossified nasal bone; 1.8% (499/27,647)
had risk more than 1:250 (screen positive). Fifteen (3.2%) of the screen positive
cases had confirmed Down syndrome while 46 had termination of pregnancy and 8 had
fetal loss.
Conclusions With the current screening protocol, the screen positive rate and the odds of being
affected given a positive result (OAPR) for a threshold of 1:250 are 1.8% and 1 in
25 respectively.
Keywords
Trisomy 21 - Down syndrome screening - India - First trimester combined screening
test - Nuchal translucency - Prenatal screening
