Objective: A study was performed to evaluate whether early second-trimester (13 to 16 weeks) transvaginal scan can be used alone for structure evaluation of the fetus or whether it should be followed by a transabdominal scan performed at 18 to 20 weeks of gestation.
Study design: A total of 536 pregnant women at high risk for birth defects were examined by transvaginal ultrasonography at 13 to 16 weeks' gestation. This was followed by a transabdominal scan at 18 to 20 weeks of pregnancy. Aborted specimens were examined by a pediatric pathologist in terminated pregnancies, and newborn babies were examined by a pediatrician to detect congenital anomalies.
Results: Transvaginal ultrasonography performed at 13 to 16 weeks' gestation identified 42 structural anomalies. Of these, 24 pregnancies were terminated and one fetus died. The other 17 fetuses and the rest of the population underwent a second transabdominal survey at 18 to 20 weeks' gestation, which identified eight structural anomalies that were not diagnosed by the previous examination. In five cases the diagnosed anomaly disappeared. Together, the scans brought about the detection of 41 of 46 abnormal fetuses (89%).
Conclusions: Because a significant number of birth defects (17.4%) was not detected by early second-trimester vaginal scan, we recommend that whenever early transvaginal ultrasonographic examination is performed it should be followed by a transabdominal scan at the most advanced stage of pregnancy at which an abortion is still feasible.