Screening for congenital heart defects by transabdominal ultrasound - role of early gestational screening and importance of operator training.

The majority of congenital heart defects occur without identifiable risk factors. Detection rates are therefore highly dependent on the experience and expertise of the obstetrical screening operator. In the first trimester, the risk of congenital heart defects increases with increasing nuchal thickness (≥2.5 mm detects 44% of major congenital heart defects), but because of the number of false positives, the positive predictive value is only a few percent. The anatomy of major congenital heart defects may be delineated in less than half of the fetuses during early second trimester. The reported yield of congenital heart defects detection during the mid-gestational routine obstetrical screening has improved over time and detection rates up to 85% of major congenital heart defects have been reported when outflow tract and three-vessel views are included in conjunction with the four-chamber view. Improved detection rates have been achieved following screening operator training interventions combined with a low referral threshold to obtain a detailed fetal echocardiographic study.