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

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Abstract
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 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.