Multiple marker screening for Down syndrome in twin pregnancies.

This study documents the screening performance, in practice, of a published protocol for interpreting second-trimester Down syndrome risk in twin pregnancies, using maternal serum biochemical markers. Within a cohort of 35,150 pregnancies, 410 twin pregnancies were identified. The rate of twinning was positively associated with maternal age. Of the 274 twin pregnancies known prior to screening, 15 (5.5 per cent) were classified as being screen-positive for Down syndrome. When maternal age and dating method were taken into account, the screen-positive rates in twin and singleton pregnancies did not differ significantly. Nine of the 14 screen-positive women with viable twin pregnancies chose amniocentesis [64 per cent, 95 per cent confidence interval (CI) 35-87]. No cases of Down syndrome were identified. Based on modelling, an estimated 73 per cent of monozygotic twin pregnancies and 43 per cent of dizygotic twin pregnancies with Down syndrome would be identified at a 5 per cent false-positive rate. The overall detection rate would be about 53 per cent. When laboratories offer multiple marker screening to women with twin pregnancies, the false-positive rate ought to be similar to that found in singleton pregnancies; the detection rate is, however, likely to be lower for dizygotic twins.