Second trimester ultrasound screening performed by midwives; sensitivity for detection of fetal anomalies

To assess the sensitivity for detection of fetal anomalies by a second trimester ultrasound screening program performed in a way representative of a majority of Swedish obstetrical departments. The examinations were performed at the ultrasound division of the Department of Obstetrics and Gynecology at the University Hospital of Uppsala, Sweden.

A prospective study covering a two-year period. Eight thousand two hundred and twenty-eight unselected, consecutive pregnant women (8345 fetuses) were examined. The ultrasound scans were performed at a gestational age of 15-22 weeks by specially trained midwives. All fetal anomalies suspected at the ultrasound screening were recorded. Follow-up of all scanned fetuses was done through neonatal reports, records from the pediatric department, a national malformation registry and autopsies. All live-born infants were followed one year after birth. Main outcome measures were sensitivity, specificity, and positive predictive value for detection of fetal anomalies and prevalence of fetal anomalies.

In all 145 fetuses/infants with confirmed anomalies were identified (prevalence 1.7%). Thirty-two were detected by second trimester ultrasound screening (sensitivity 22.1%). Twenty false positive cases were identified, of which fifteen were ruled out on the same day the suspicion arose and five were transient findings. The specificity was 99.8% and the positive predictive value 61.5%

When performing a second trimester ultrasound screening program, prenatal diagnosis of fetal anomalies will be part of the procedure whether this is a primary aim or not. It is therefore of great importance that the pregnant women receive adequate information and that the voluntariness of participation is stressed. In this study the sensitivity was low. Standardizing the scanning procedure (check-list for fetal anatomy) and improving the education of the operators are feasible ways to increase the sensitivity.