Effects of routine one-stage ultrasound screening in pregnancy: a randomised controlled trial.

4997 of 7354 pregnant women had no clinical indication for an elective ultrasound examination at 12 weeks' gestation. 2482 of these women were randomly selected for ultrasound screening at 15 weeks and the remainder received the same standard antenatal care without the scan. Labour was less often induced among screened women both for all reasons (5.9% vs 9.1%, p less than 0.0001) and for suspected post-term pregnancy (1.7% vs 3.7%, p less than 0.0001). Earlier detection of twins had no effect on neonatal outcome. Among babies born to screened women, fewer were of birthweight less than 2500 g (59 vs 95, p = 0.005) and mean birthweight was 42 g higher (p 0.008). For babies born to screened women who smoked it was 75 g higher (p 0.012) and for those of non-smokers 26 g (not significant). The reason for the differences in mean birthweight could be that screened women reduced smoking in response to watching their fetus on the scan.