Performing third-trimester ultrasound at 30-32 weeks, the rate of antenatal detection of fetal growth restriction is 20.5% in low risk pregnancies. Aiming to increase the detection rate by at least 7%, the investigators will require a total sample of 1,200 pregnant women (600 in each group), with 80% power and α level of 0.05. Thus an interim analysis is expected to be performed when 1200 cases are recruited to the study to compare the detection rate of fetal growth restriction between the two groups.