Physiologic restriction versus genetic weight potential: study in normal fetuses and in fetuses with intrauterine growth retardation.

Physiologic weight restriction is defined as the difference between the genetic and real weight in a normal fetus. The aims of this study were (1) to obtain, in normal pregnancies, reference values of mean weight restriction between 32 and 42 weeks for both male and female fetuses, and (2) to observe how weight restriction may influence intrauterine growth retardation. In the first part of the study, 1004 ultrasonograms of 389 different women were studied and the estimated fetal weights with their regression curves were calculated and drawn for all fetuses by sex. Differences between the 50th percentile of the genetic curves in normal population and the estimated fetal weight values for each of the 1004 examinations were calculated and weight restriction 50th and 90th percentiles were described. In the second part of the study, genetic curves were constructed retrospectively for 20 fetuses with intrauterine weight restriction whose examinations were performed before week 28 and were compared with curves for the normal population. Finally, for the 20 patients with intrauterine weight restriction, differences between genetic and real weight at delivery were plotted and compared with weight restriction 50th and 90th percentiles. Also, fetuses with intrauterine weight restriction were compared according to differing degrees of restriction. Weight restriction began between 31 and 33 weeks of gestation and was earlier and marked in female fetuses. Genetic percentiles were higher in normal fetuses than in fetuses with intrauterine weight restriction. In addition, pregnancies of intrauterine growth restricted fetuses with greater degrees of weight restriction were more abnormal than those of fetuses with a lesser degree of weight restriction. Both facts imply that some of the fetuses included in the classic diagnosis of intrauterine weight restriction may be genetically small fetuses. Concepts of weight restriction and physiologic weight restriction might be applied to discriminate between normal, genetically small fetuses and fetuses affected with intrauterine growth retardation.