Ultrasound screening for fetal growth restriction at 36 vs 32 weeks' gestation: a randomized trial (ROUTE)

To compare the utility of routine third-trimester ultrasound examination at 36 weeks' gestation with that at 32 weeks in detecting fetal growth restriction (FGR).

This was an open-label parallel randomized trial (ROUTE study) conducted at a single general hospital serving a geographically well-defined catchment area in Barcelona, Spain, between May 2011 and April 2014. Women with no adverse medical or obstetric history and a singleton pregnancy without fetal abnormalities at routine second-trimester scan were assigned randomly to undergo a scan at 32 weeks' gestation (n = 1272) or at 36 weeks' gestation (n = 1314). Primary outcome measures were detection rates of FGR (customized birth weight < 10(th) centile) and severe FGR (customized birth weight < 3(rd) centile).

There were no significant differences in perinatal outcome between those who underwent a scan at 32 weeks' gestation and those who underwent a scan at 36 weeks' gestation. Severe FGR at birth was associated significantly with emergency Cesarean delivery for fetal distress (odds ratio (OR), 3.4 (95% CI, 1.8-6.7)), neonatal admission (OR, 2.23 (95% CI, 1.23-4.05)), hypoglycemia (OR, 9.5 (95% CI, 1.8-49.8)) and hyperbilirubinemia (OR, 9.0 (95% CI, 4.6-17.6)). Despite similar false-positive rates (FPRs) (6.4% vs 8.2%), FGR detection rates were superior at 36 vs 32 weeks' gestation (sensitivity, 38.8% vs 22.5%; P = 0.006), with positive and negative likelihood ratios of 6.1 vs 2.7 and 0.65 vs 0.84, respectively. In cases of severe FGR, FPRs for both scans were also similar (8.5% vs 8.7%), but detection rates were superior at 36 vs 32 weeks' gestation (61.4% vs 32.5%; P = 0.008). Positive and negative likelihood ratios were 7.2 vs 3.7 and 0.4 vs 0.74, respectively.

In low-risk pregnancies, routine ultrasound examination at 36 weeks' gestation was more effective than that at 32 weeks' gestation in detecting FGR and related adverse perinatal and neonatal outcomes.