Procollagen propeptide serum levels reflect the rate of collagen production. Because the prenatal period is unrivaled in terms of relative amounts of collagen synthesized per unit of time, this life episode must be the most sensitive period for the study of these biochemical markers of growth variability. N-terminal propeptide of type III procollagen (PIIINP) concentration was measured by two different methods (Fab'-fragment and whole antibody-based RIA assay) on paired samples of cord serum and amniotic fluid from a study cohort of 602 perinates with gestational age ranging from 20 to 41 wk. The aim of this study is to assess the clinical usefulness of the PIIINP assay for the evaluation of fetal somatic growth variation during the second half of normal pregnancies. It was demonstrated that the PIIINP level in cord serum, as well as in amniotic fluid, reflects age-related growth activity in "healthy" fetuses (n = 504) with normal intrauterine growth. This reflection was independent of the cumulative body mass or length already attained at the time of investigation. The PIIINP concentration closely mirrors the shape of the fetal somatic growth velocity curve, expressed as weight-specific gain (g/kg/d) during the second half of pregnancy. It can be concluded that PIIINP level in cord blood or amniotic fluid of fetuses with normal intrauterine growth is an interesting parameter for the assessment of maturity-related fetal growth potential.