The aim of this study was to relate human fetal growth retardation to specific hormone alterations. Serum levels of GH, IGF-I, IGF-II, and IGF binding protein (BP) 3 were measured during the second half of gestation after cordocentesis in 230 fetuses who were classified into normally grown (n = 166) and growth-retarded (n = 64) groups according to ante- and neonatal measurements. The normally grown group showed a progressive decline in serum GH levels toward term (r = -0.42, p = 0.0001), whereas serum IGF-I was increased (r = 0.55, p = 0.0001), as were serum IGF-II (r = 0.21, p = 0.008) and IGFBP3 levels (r = 0.19, p = 0.02), although less markedly. For all hormone levels, wide individual differences were found at any given age of gestation. The incidental presence of fetal malformations in either the normally grown group (n = 107 cases) or the growth-retarded group (n = 50 cases) had no apparent effect on these hormone levels as compared with members of the groups showing no fetal malformations (n = 73 cases). Comparison of the normally grown group with the growth-retarded group showed that serum IGF-I levels were significantly lower in the growth-retarded group (p = 0.001). No differences were found between the groups in serum GH, IGF-II, and IGFBP3 levels, although if data for the third trimester were taken alone, serum IGF-II levels were found to be lower in the growth-retarded group (p = 0.05). In conclusion, during the second half of gestation, fetal serum IGF-I levels may be influenced by nutritional factors controlling fetal growth. However, the wide individual differences in measurements make it a very poor biologic marker of intrauterine growth retardation.