Serum concentrations of insulin-like growth factors and placental lactogen during gestation in cattle. I. Fetal profiles.

Sixty crossbred beef heifers pregnant with fetuses of either high (H; n = 30) or low (L; n = 30) genetic potential for growth were used to establish fetal serum profiles of insulin-like growth factor (IGF)-I, IGF-II, and placental lactogen (PL), and to examine relationships between serum hormone concentrations and fetal growth parameters. Three H and three L fetuses were collected by caesarean section at 21-d intervals from Day 85 through Day 274 of gestation. Arterial, venous, and mixed umbilical blood samples were collected during surgery. Fetal body weight, crown-rump length, hip height, and heart girth were measured. Serum concentrations of IGF-I and IGF-II increased (P < 0.0001) with advancing gestational age, whereas serum PL declined (P < 0.0001) linearly (P < 0.01) throughout gestation. Serum concentrations were greater in the umbilical vein compared with the umbilical artery for IGF-II (P < 0.0001) and PL (P < 0.05), but not IGF-I. Fetal IGF-I, IGF-II, and PL serum concentrations were not correlated with serum concentrations of the respective maternal hormones. Fetal serum IGF-I concentrations were correlated to fetal body weight (r = 0.66), growth rate (r = 0.72), crown-rump length (r = 0.20), hip height (r = 0.17), and heart girth (r = 0.20). Correlations between fetal serum IGF-II concentrations and the same parameters were 0.60, 0.62, 0.39, 0.34, and 0.37, respectively. Fetal serum PL concentrations were negatively correlated to body weight (r = -0.40) and growth rate (r = -0.40) and not correlated with any fetal linear measure. Fetal genotype (L vs. H) differences were detected for IGF-I (P < 0.05) and PL (P = 0.09) concentrations. Fetal sex effects were not observed for any hormone. Maternal sire breed, hip height, and body condition score did not influence fetal serum hormone concentrations.