Additional single third trimester ultrasound scan in detection of large for gestational age fetuses

To evaluate the accuracy of growth scans in the third trimester. To evaluate the accuracy of universal third trimester ultrasound scans in the detection of large for gestational age (LGA) fetuses. To investigate how universal ultrasound scans affect the delivery and outcomes.

Universal third trimester screening for LGA recorded a 22% positive predictive value (PPV), and 96% negative predicted value (NPV). The sensitivity in most studies reaches around 70% or more, of all the LGA fetuses delivered, there are different studies for and against universal ultrasonic screening of LGA. Estimated fetal weight (>4 kg/90th centile) and abdominal circumference are the best predictors of LGA with nearly 70% sensitivity. One study reported that an antenatal ultrasonic diagnosis of LGA in a low risk population has a weak association with the incidence of shoulder dystocia or poor neonatal outcomes. Universal screening in the third trimester for LGAs is not cost effective.

Low risk pregnancies constitute the majority of the pregnancy population. All low risk pregnancy women will only receive two routine scans throughout the whole pregnancy. There is no evidence at present that conclusively demonstrates that an additional scan at 36 weeks improves maternal and neonatal outcomes and is cost effective.