Objective An increasing number of ultrasound departments, often under substantial workload pressures, have stopped scanning for ‘growth’ at term, claiming that ultrasound biometry is not accurate in late pregnancy. We wanted to test this hypothesis, comparing the results of scan estimated fetal weight (EFW) with the baby's subsequent weight at birth, for preterm versus term gestations.

Method The data were derived from a regional database of routinely collected information from 19 regional maternity units, and included dates and measurements of ultrasound scans and the date and weight of the baby at birth. An ultrasound scan was performed within 3 days of delivery in 2296 pregnancies, including 606 preterm (<37 weeks) and 1690 term deliveries. EFW was calculated by standard Hadlock formulae programmed into the respective units' scanners. Presumed weight gain during the 1, 2 or 3 day delay between scan measurement and birth was adjusted for by a previously described ‘proportionality’ formula.

Results Table 1 gives the proportion of cases where the scan EFW was within a 10%, 15% or 20% error margin of the birthweight. Overall, EFWs at term were at least as good and in fact marginally better than scans done in the preterm period, with 73% of EFWs falling within a +/−10% margin of error.

Conclusion Ultrasound biometry is an accurate means to ESW at term as well as preterm gestations. It is an essential tool in efforts to identify the pregnancy at risk for fetal growth restriction