Differences in size at birth are determined by differences in growth velocity during early prenatal life

Physiologic interindividual differences in neonatal size are traditionally thought of as determined by differences in fetal growth occurring only in the second half of pregnancy. Whether possible differences in early intrauterine growth velocity are the effect of random growth fluctuations or may affect size at birth is still debated. This article aims at evaluating to what extent differences in neonatal size are accounted for by differences in fetal growth velocity. We analyzed the fetal growth of 130 healthy singletons for whom head (HC) and abdomen (AC) circumferences and femur diaphysis length (FDL) longitudinal profiles were available, together with the measures of weight (BW), length (BL), and head circumference (BHC) at birth. Individual profiles were fitted with ad-hoc models. Neonatal traits were transformed into standard deviation scores (SDS). Neonates in the upper third of BW-SDS distribution (3618+/-43 g, mean+/-SEM) had, at 22 wk of gestational age, AC growth velocity higher by 0.55+/-0.10 mm/wk than those in the lower third (2902+/-36 g). Neonates in the upper third of BL-SDS distribution (51.7+/-0.21 cm) had, at 20 wk, FDL growth velocity higher by 0.11+/-0.05 mm/wk than those in the lower third (48.2+/-0.18 cm). Neonates in the upper third of BHC-SDS distribution (35.7+/-0.13 cm) had, at 18 wk, HC growth velocity higher by 0.57+/-0.20 mm/wk than those in the lower third (33.3+/-0.11 cm). The differences in growth velocity remain constant throughout the second and third trimester for AC, and tend to vanish in the third trimester for HC and FDL. The differences in fetal growth velocity, which in our study were observed as early as mo 4, suggest that the genetic component plays an important role in fetal growth and is precociously expressed.