Prediction of risk for vasa previa at 9-13 weeks' gestation.

To assess the usefulness for predicting vasa previa by detecting a cord insertion site in the lower third of the uterus between 9 and 13 weeks' gestation. The positional relationship between the uterine cavity and the cord insertion site was examined in consecutive subjects prospectively using ultrasonography at 9-13 weeks' gestation. The distance between the internal os and the fundus was divided equally into three parts. Cord insertions in the lower third were defined as cases; other insertions were defined as controls. Placental and umbilical cord abnormalities at the time of the delivery were analyzed between the two groups. The cord insertion sites were identified as 139 (10.6%) cases with low cord insertion and 1172 control cases. The case subjects frequently had an abnormal placental form (6.5% vs 2.1%, RR 3.2, 95% CI 1.5-7.0) or placenta previa (4.7% vs 1.3%, RR 3.5, 95% CI 1.3-9.1). The frequencies of velamentous cord insertion were 7.2% in cases and 0.9% in controls (RR 8.1, 95% CI 3.4-19.6). Three cases (2.2%) of vasa previa were observed in the cases, but none were observed in the controls (P = 0.001). Placental abruption occurred in 4.3% of the cases and 0.9% of the controls (RR 4.7, 95% CI 1.7-13.1). Screening sonography in the late first or early second trimesters and following up at the second trimester in cases with low cord insertion is a useful way to detect vasa previa.