Routine versus indicated third trimester ultrasound: is a randomized trial feasible?

An adequate and contemporary randomized trial is needed to resolve whether routine third trimester ultrasound followed by adapted perinatal management improves perinatal outcomes in a population of women at low risk. We aimed to describe current practices regarding third trimester ultrasound in our centre and to evaluate the feasibility of a randomized trial.

All women with a singleton pregnancy managed from the beginning of pregnancy in our maternity unit who delivered after 28 weeks (N = 335) were assessed prospectively over a 50-day period. Details of maternal characteristics, medical and obstetrical history, management of pregnancy and delivery, ultrasound practices, and results were recorded. One hundred women had a brief personal interview to define their expectations and experience of third trimester ultrasound.

The women who were assessed had 2.9 +/- 1.2 ultrasound scans during their pregnancy. All had a second trimester ultrasound scan and 53.7% had a third trimester scan. There was no medical indication for the third trimester ultrasound scan in 12.8% of the women. Among women with a low-risk pregnancy, 40% had a third trimester ultrasound, and 21.6% of those were done without medical indication. Among women with a low-risk pregnancy who had a third trimester ultrasound, the interview disclosed that 80% found that the test was not stressful. Of the low-risk population interviewed, 83.6% would agree to participate in a future trial.

Although any study designed to evaluate the effect of routine third trimester ultrasound on perinatal morbidity and mortality in a low-risk pregnant population would include a large patient sample, our study shows that a randomized trial is feasible because most women with a low-risk pregnancy do not consider this examination stressful and would volunteer to participate.