Longitudinal distance standards of fetal growth

Background. Most ultrasonographic fetal growth norms are derived from cross -sectional data or from longitudinal data treated as coming from cross-sect ional studies, although only longitudinal models may detect particular aspe cts of fetal growth shape, such as peak of growth velocity. Materials and methods. The sample included 238 singleton normal pregnancies . All the fetal traits under study (biparietal diameter, occipito-frontal d iameter, head circumference, femur diaphysis length and abdomen circumferen ce) were measured according to the classical ultrasound techniques by highl y trained operators. Individual growth profiles (made up of 5 to 9 measures ) were taken at regular intervals between the 12th and the 40th week. Growt h norms were traced by means of a two-stage linear model: (I) a 3-constant fetal growth function was fitted to each individual growth profile, (II) gr owth centiles were based upon the weighted mean and covariance matrix of th e individual growth constants. Results. Fetal growth curves show a sigmoid shape with a maximum slope (i.e . a peak growth velocity) which occurs earlier for head diameters (about 18 weeks), later for femur diaphysis length (20 weeks) and abdomen circumfere nce (22 weeks). During intrauterine growth, all traits show a progressive i ncrease in interindividual variability, which is more prominent for abdomen circumference. Conclusion. The mathematical model applied to a large sample of growth prof iles provided a satisfactory description of the individual fetal developmen t and its biological variability, and allowed the construction of longitudi nal distance standards useful. for clinical purposes.