Learning curve in ultrasonographic screening for selected fetal structural anomalies in early pregnancy

To assess the value of first trimester screening by ultrasonography in detecting structural anomalies of the fetus in a general obstetric population.

During 1993-1998, 20,465 consecutive pregnant women who resided in a defined geographic area participated in ultrasonographic screening for major malformations. These included anomalies of the central nervous system, urinary tract, abdominal wall, and long bones. Heart anomalies were not expected to be detected. The examinations were offered at 13-14 weeks' gestation as part of routine maternal care and were done by specially trained midwives. The pregnancy outcomes were ascertained from obstetric and pediatric records, and the data were completed by information from the national birth and malformation registries.

A total of 307 fetuses (1.5%) with a major malformation were found; 67 fetuses (0.3%) had noncardiac major structural defects expected to be detectable by ultrasonography in early pregnancy. Thirty-five of 67 (52%) were identified at the early scan. Sensitivity for these defects increased from 22% to 79% from the first to the last (sixth) study year (P =.009).

In a low-risk population, adequate sensitivity in screening for major malformations by early ultrasonography can be achieved after a learning curve of 3-4 years.