The aim of the study was 1) to evaluate the association of maternal serum levels of placental GH and IGF-I with fetal growth, and 2) to establish reference data for placental GH, IGF-I, and IGF-binding protein-3 (IGFBP-3) in normal pregnancies based on longitudinal measurements. A prospective longitudinal study of 89 normal pregnant women was conducted. The women had, on the average, seven blood samples taken and three ultrasound examinations performed. All had normal umbilical artery pulsatility indexes during pregnancy and gave birth to singletons between 37 and 42 wk gestation with birth weights above -2 SD. Placental GH levels were detectable in all samples from as early as 5 wk gestation and increased significantly throughout pregnancy to approximately 37 wk when peak levels of 22 ng/ml (range, 4.64-69.22 ng/ml) were reached. Subsequently, placental GH levels decreased until birth. The change in placental GH during 24.5-37.5 wk gestation was positively associated with fetal growth rate (P = 0.027) and birth weight (P = 0.027). Gestational age at peak placental GH values (P = 0.007) was associated with pregnancy length. A positive association between the change in placental GH and the change in IGF-I levels throughout gestation was found in a multivariate analysis (r(2) = 0.42; P < 0.001). There was no association between placental GH and IGFBP-3 levels. The change in IGF-I throughout gestation (P = 0.039), but not placental GH, was significantly positively associated with placental weight at birth. We found a significant association between placental GH and fetal growth. In addition, we found a highly significant association between the increase in placental GH and the increase in IGF-I. The gestational age at peak placental GH levels was associated with pregnancy length.