Detection of malformations in chromosomally normal fetuses by routine ultrasound at 12 or 18 weeks of gestation - a randomised controlled trial in 39 572 pregnancies

Objective To compare the antenatal detection rate of malformations in chromosomally normal fetuses between a strategy of offering one routine ultrasound examination at 12 gestational weeks (gws) and a strategy of offering one routine examination at 18 gws. Design Randomised controlled trial. Setting Multicentre trial including eight hospitals. Population A total of 39 572 unselected pregnant women. Methods Women were randomised either to one routine ultrasound scan at 12 (12-14) gws including nuchal translucency (NT) measurement or to one routine scan at 18 (15-22) gws. Anomaly screening was performed in both groups following a check-list. A repeat scan was offered in the 12-week scan group if the fetal anatomy could not be adequately seen at 12-14 gws or if NT was >= 3.5 mm in a fetus with normal or unknown chromosomes. Main outcome measures Antenatal detection rate of malformed fetuses. Results The antenatal detection rate of fetuses with a major malformation was 38% (66/176) in the 12-week scan group and 47% (72/152) in the 18-week scan group (P = 0.06). The corresponding figures for detection at < 22 gws were 30% (53/176) and 40% (61/152) (P = 0.07). In the 12-week scan group, 69% of fetuses with a lethal anomaly were detected at a scan at 12-14 gws. Conclusions None of the two strategies for prenatal diagnosis is clearly superior to the other. The 12-week strategy has the advantage that most lethal malformations will be detected at < 15 gws, enabling earlier pregnancy termination. The 18-week strategy seems to be associated with a slightly higher detection rate of major malformations, although the difference was not statistically significant.