Are Neonatal Growth Standards Adequate to Diagnose Fetal Growth Restriction in Preterm Babies?

Introduction: Growth standard derived from birth weight at different gestational ages (neonatal curves) probably underestimate small-for-gestational-age (10th percentile) prevalence in comparison with ultrasonographically determined fetal growth standards. The aim of this observational study was to compare local neonatal standards and estimated fetal weight using different models at similar gestational ages. Material and Methods: 55706 singleton births between 25 and 42 weeks of gestation born at Maternidad Sardá, Buenos Aires from 1988 to 1999, were included (preterms –25th to 36th weeks-, n = 3745). Fractional growth curve was built using Mongelli′s formula, which for any gestational age, yields fetal weight as a percentage of the final term weight. Percentage differences were calculated at 10th and 50th percentile levels between observed weight (neonatal curve) and estimated weight (fractional curve). These differences were then compared with the 95% CI (absolute mean error of estimated weight). Comparisons were made among Maternidad Sardá′s fractional curve, a similar one from Notingham (UK) and the fetal ultrasonographic standard from Hadlock. Results: Neonatal weights were significantly lower than those derived from fractional curve for percentiles 10th and 50th. At percentile 10th, weights calculated from neonatal standards during the premature period were significantly lower than those estimated ultrasonographically, while Sardá fractional curve was slightly above Nottingham′s. Percent differences between neonatal and predicted weights were highly statistically significant during the premature period, being consistently higher for the 10th percentile (p = 0.009). Differences at 10th percentile were in a systematic way higher than the upper limit of the 95% CI. Conclusions: Local neonatal standard based on birth weight for gestational age, probably underestimate fetal growth and therefore underestimate the proportion of prematures which are small-for-gestational-age.