Intra-uterine growth: a comparison of longitudinal and cross-sectional approaches.

The great majority of the fetal growth curves are constructed through a cross-sectional approach. Before the use of ultra-sonography, fetal growth was first evaluated from birthweights of infants born at different gestational ages. More recently, curves were established for different ultrasonographic parameters which combination can provide an immediate but imperfect evaluation of fetal weight. All the curves used as standards of normal growth derive from cross-sectional studies. Several longitudinal studies of fetal growth exist, with two aspects. The first method is to make serial measurements at special intervals, among a group of fetuses with known dates of conception, in order to determine average longitudinal curves. The aim of the standards of fetal growth is obstetric management and the threshold of a percentile, for instance the tenth, of an ultra-sonographic parameter can define the intra-uterine growth retardation. The result of the fetal growth is evaluated with the standards of birthweight which define the small for gestational age infant. Another longitudinal approach is the use of a mathematical growth model to establish an individual growth curve, each fetus is then its own control. This last approach still belongs to the research field.