Are there risk factors for false-positive malformation diagnoses on obstetric ultrasound? A nested case-control study

In a population-based study, we found an overall false-positive rate of 8.8% for the second and third trimester ultrasounds. Although numerous studies have been performed to examine factors which lead to false negatives, the same is not true for the factors associated with false positives. The principal objective of this study was to look for risk factors for false-positive diagnoses of fetal malformations on obstetric ultrasound scans.

In this nested case-control study, the case infants were those whose mother had a false-positive antenatal ultrasound diagnosis of a malformation during the second or third trimester (ultrasound false-positives) and who were live - or stillborn in Auvergne in 2006-2010. The control group comprised all children who were ultrasound true-negatives in 2005 and 2007. The study included 46 cases and 184 controls, matched according to the level of the maternity unit in which they were born.

Most false-positive diagnoses were minor malformations. The mean term at this false-positive diagnosis was 27.7±5.4 weeks; in 46.8% of cases, the diagnosis was made during the second-trimester ultrasound. A single malformation was suspected in 95.7% of the cases. In 97.9% of cases, only one anatomical system was affected. In all, 49 malformations were identified among the 46 cases and their distribution differed according to anatomical system. The only risk factor identified was a body mass index (BMI)<25 (ORa=1.7; 95%CI: 1.2-2.4).

A maternal BMI<25 was the only risk factor identified for a false-positive ultrasound diagnosis of a fetal malformation.