Two‐stage ultrasonography in screening for fetal anomalies at 13–14 and 18–22 weeks of gestation

Background.  The aim of this study was to assess the value of two‐stage screening by ultrasonography in detecting selected major fetal anomalies in a low‐risk obstetric population. Methods.  In a defined geographic area, 4789 consecutive low‐risk pregnant women participated in screening by two‐stage ultrasonography as part of routine maternal care. The examinations were usually performed by specially trained midwives at 13–14 and 18–22 weeks of gestation. Of the women, 4073 had both scans, 440 had the early one only, and 276 the late scan only. Pregnancy outcomes were ascertained from obstetric and pediatric records, and the data were supplemented with information from the national birth and malformation registries. Results.  Of the 4855 fetuses, 33 (0.7%) had major structural defects considered detectable by ultrasonography. Of these, six (18%) were identified at the early scan, and an additional 10 (30%) at the late scan, yielding a total sensitivity of 48% for the two‐stage screening. Twenty offspring had chromosomal abnormalities; 10 were identified by increased nuchal translucency at the early scan, one additional one (by hydronephrosis) at the late scan, and the remaining nine at birth. Conclusions.  In a low‐risk population, first‐trimester scanning is useful in finding fetuses with chromosomal anomalies, but a second‐trimester scan is needed for other types of defects. The sensitivity of routine screening by midwives for fetal structural defects in a general obstetric population remains lower than that reported by specialized centers.