False positives in the prenatal ultrasound screening of fetal structural anomalies.

OBJECTIVE: To describe the false-positive diagnoses of prenatal ultrasound screening of fetal structural anomalies. METHODS: Pregnancies with fetal structural anomalies either detected prenatally in our center or referred to us, were registered, evaluated, and followed-up prospectively by a multidisciplinary Congenital Defects Committee. After postnatal follow-up was completed, cases were assigned as true positives, false positives or false negatives and categorized by anatomical systems. Pregnancies referred with a nonconfirmed suspicion of anomaly were not included. The false-positive diagnoses were analyzed. RESULTS: From 1994 to 2004, 903 new registry entries of fetuses structurally abnormal at ultrasound with a complete follow-up were included in the Committee database. There were 76 false positives, accounting for 9.3% of all the prenatally established diagnoses. The urinary tract anomalies were the most frequent false-positive diagnoses found (n = 25; accounting for 8.7% of the urinary tract defects), but the genital anomalies showed the higher rate of no confirmation (n = 5; 15.2%). The specific anomalies most commonly not confirmed were renal pyelectasis (n = 9), cerebral ventriculomegaly (n = 9), abdominal cysts (n = 7) and short limbs (n = 7). CONCLUSION: Several prenatally diagnosed anomalies would benefit from prudent counseling, because they may be normal variants or transient findings.