OBJECTIVE: To evaluate differences in distribution of estimated fetal weight (EFW) and birth weight (BW) of ongoing fetuses and neonates of the same gestational age.METHODS: Reference curves for EFW (Hadlock BPD-HC-AC-FL formula, N = 1191) and BW (N = 1036) in singleton pregnancies from 24 to 40 gestational weeks were calculated. Multiple pregnancies, fetuses with major or multiple abnormalities or syndromes and iatrogenic preterm deliveries due to preeclampsia or abnormal fetal Doppler were excluded. The standardized residuals for EFW and BW were calculated and compared.RESULTS: EFW and BW can be accurately described by quadratic equations (R = 0.944 and 0.807, respectively). The distribution of standardized residuals for BW using the EFW formula was negative from 28 to 35 weeks. The 50th and 5th centiles of BW were lower than those of EFW throughout prematurity, and they converged at approximately 38 gestational weeks. The 5th centile for BW was 30% lower than the 5th centile for EFW at 27 weeks, 27.5% lower at 30 weeks and 19.4% at 34 weeks.CONCLUSIONS: Preterm infants have lower BW distribution compared to the expected EFW of ongoing pregnancies of the same gestational age, supporting the concept of hidden intrauterine morbidity for a proportion of these infants.