Compromise ultrasound dating policy in maternal serum screening for Down syndrome

Routine ultrasound biometry is the method of choice for gestational dating when screening for Down syndrome. However, it is costly and an alternative policy is to restrict ultrasound to women most likely to have menstrual dating errors. This was evaluated by statistical modelling with parameters from 14,274 women screened between January 1997 and July 2001 using free beta-human chorionic gonadotrophin (free beta-hCG), alpha-fetoprotein (AFP) and unconjugated estriol (uE(3)). A total of 12,711 (89%) women had both ultrasound and menstrual gestations, but in 4101 (29%) women either the last menstrual period (LMP) was uncertain or a pill-withdrawal period, or there were irregular or abnormal length cycles. The LMP was not entered in the test request form for a further 1404 (9.8%) women. Routine ultrasound dating yielded a predicted detection rate higher than for menstrual dating by 3.9-7.1%, depending on the marker combination and cut-off. The false-positive rate was reduced by 0.2-1.1%. Selectively scanning the 39% with unreliable dates increased detection by 2.6-4.6%, and reduced the false-positive rate by 0.04-0.6%. Some centres only use the ultrasound estimate of gestation when it differs from the menstrual estimate by more than 7 days. Such a rule reduces the gain in detection rate to 2.5-4.6% for routine ultrasound and 1.7-3.1% with the compromise policy; the false-positive rate reductions are 0.06-0.6% and 0.0-0.3%, respectively. We conclude that if routine ultrasound is not financially and practically feasible, the compromise policy yields a clinically important improvement in screening performance compared to menstrual dating.