A randomized controlled trial of third-trimester routine ultrasound in a non-selected population

Abstract

Objective: To compare detection rates of small-for-gestational-age fetuses, large-for-gestational-age fetuses, congenital anomalies and adverse perinatal outcomes in pregnancies randomized to third-trimester routine ultrasound or ultrasound on clinical indication.

Design: Randomized controlled trial.

Setting: National Center for Fetal Medicine in Norway between 1989 and 1992.

Population: A total of 6780 pregnancies from a non-selected population.

Methods: Two routine ultrasound examinations at 18 and 33 weeks were compared with routine ultrasound at 18 weeks and ultrasound on clinical indication. Suspected small-for-gestational-age fetuses were followed with serial scans and cardiotocography. Doppler ultrasound was not used.

Main outcome measures: Detection rates of small-for-gestational-age and large-for-gestational-age fetuses, congenital anomalies and adverse perinatal outcomes.

Results: Third trimester routine ultrasound improved detection rates of small-for-gestational-age fetuses from 46 to 80%, but overall perinatal morbidity and mortality remained unchanged. Detection of large-for-gestational-age fetuses increased from 36 to 91%. There was a significant increase of induction of labor and elective cesarean sections due to suspected small-for-gestational-age and a significant decrease of induction of labor and elective cesarean sections due to suspected large-for-gestational-age in the study group; there were no other differences regarding intervention. The detection rate of congenital anomalies was 56%, with no significant difference between the groups.

Conclusions: Routine use of third-trimester routine ultrasound increased detection rates of small-for-gestational-age and large-for-gestational-age fetuses. This did not alter perinatal outcomes. Third-trimester ultrasound screening should not be rejected before a policy of adding Doppler surveillance to the high-risk group identified has been investigated further.

Keywords: Pregnancy; intrauterine growth restriction; large-for-gestational-age; perinatal outcome; routine ultrasound; small-for-gestational-age; third trimester.