To compare the accuracy of estimated fetal weight (EFW) in extremely preterm small for gestational age (SGA) and appropriate for gestational age (AGA) infants and report other significant factors influencing the accuracy of EFW.

A retrospective cohort study of singleton pregnancies 22+0–27+6 weeks. Women were included in the study if an ultrasound scan had been performed within seven days of delivery, with no major fetal anomaly and data available to calculate customised birthweight (BW) centiles. Mean error of EFW and actual BW and mean % error of EFW and actual birthweight were compared for SGA and AGA infants. A stepwise backward elimination linear regression model was used to determine the significant factors influencing the accuracy of EFW.

A total of 134 cases (51 SGA and 83 AGA) were analysed. The mean gestational age at delivery was 25+2 weeks (SD 11.5 days) and mean BW 711 g (SD 227 g). Overall mean percentage error of EFW and actual BW was 8.8% (range 0–34.6%). There was a significant difference in mean error of EFW and actual BW for SGA and AGA deliveries (mean +16 g versus −23 g, respectively, P = 0.01) and in mean % error of EFW (11.2%, 95%CI 9.1–13.3 versus 7.4%, 95% CI 6.2–8.6 P = 0.009). Factors that significantly influenced the accuracy of EFW included SGA (P = 0.001, coeff. = −3.73, 95% CI −5.94/–1.52), scan to delivery interval (P = 0.02, coeff. = 0.66, 95% CI 0.12/1.21) and reduced amniotic fluid (P = 0.008, coeff = 3.61, 95% CI −5.47/−0.85).

Ultrasonographic EFW for extreme preterm SGA fetuses is less accurate than AGA fetuses and is more likely to overestimate EFW. This should be considered when counselling women with growth restricted fetuses at the limits of viability.