OBJECTIVE To estimate the detection rate of abnormalities by transvaginal ultrasound in early pregnancy. METHODS We prospectively analyzed records of 3592 sequential pregnant women at 10-16 weeks' (singleton) gestation (mean 13 weeks and 2 days). After exclusion of 114 women, there were 3478 women in the study. Each woman underwent a transvaginal sonographic survey for fetal anomalies as well as biometric measurements. Fetuses diagnosed with malformations were followed to delivery, and those without underwent transabdominal sonography at 18-24 weeks' gestation. RESULTS The anomaly detection rate by transvaginal ultrasound was 51.6% (33 of 64; 95% confidence interval [CI] 38.7, 64.2) in early pregnancy, and the detection rate by transvaginal ultrasound combined with second-trimester transabdominal ultrasound was 84.4% (54 of 64; 95% CI 73.1, 92.2). Cystic hygroma and fetal hydrops were the anomalies detected most frequently by transvaginal ultrasound. Low detection rates for abnormalities of the face and of the cardiac, skeletal, and urinary systems were found even when both methods were used. CONCLUSIONS Transvaginal sonography appears to be an effective way to identify many congenital fetal anomalies in early pregnancy. There is a good probability of diagnosing cystic hygroma and fetal hydrops, although other abnormalities, particularly heart defects, are associated with lower detection rates.