It is well known that the biologic variability in fetal size increases as pregnancy advances, although the embryonal and early fetal growth patterns as well as how early and how much the genetic, hormonal and environmental variables play a role in its modulation are still debated. It is accepted that growth in the first trimester of pregnancy is relatively uniform, with a minimal biologic variability; this variability may be underestimated, because the transversal studies do not permit the identification of the growth pattern. The aim of this work is to evaluate, by means of a longitudinal study, the time of embryo-fetal growth differentiation related at neonatal anthropometric measurements. We evaluated 238 neonates (123 female; 115 male) delivered at term after low risk, uncomplicated pregnancies. The subjects were divided into three tertles (low, mid and high) according to birth weight, length and head circumference. For each tertle, distance curves, velocity curves, and rate of increase were calculated by using respectively fetal abdominal circumference (for birth weight), fetal femural length (for neonatal length) and fetal head circumference (for neonatal circumference). The distance curves showed clear differences among the tertles only in the second period of pregnancy, whereas the velocity curves showed clear differences among tertles already in the first 12 weeks. The value of growth rates were similar for all the variables during the entire time considered. This study shows that the anthropometric differences between newborn subgroups exist already at the end of the first trimester of pregnancy and, in physiological conditions, until the end of pregnancy. The anthropometric differences observed early in our study, at twelve weeks of gestational age, are still present at the end of pregnancy and let us suppose a very early expression of the genetic potential for individual growth.
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