The relationship of body and placental weight to plasma levels of insulin and other hormones during development in fetal rabbits.

The relationship of body and placental weight with plasma levels of insulin, glucose, glucagon, glucocorticoids and somatomedins in fetal rabbits between 22 days of gestation and term has been investigated. At 22, 24, 26, 28 and 30 days gestation, body and placental weights were strongly correlated and were also significantly correlated with log transformed plasma insulin concentrations in individual fetuses from nulliparous does. At 30 days gestation, mean plasma insulin levels, as well as body and placental weights, were significantly elevated in fetuses from multiparous does (p less than 0.05) and in fetuses whose growth had been increased by surgical reduction in litter size on day 9 of pregnancy (p less than 0.001). Log insulin concentrations in fetuses from multiparous does and in litter-reduced fetuses were also significantly correlated with bodyweight. The correlation coefficient for log insulin and bodyweight in all individual rabbit fetuses at 30 days gestation was 0.69 (p less than 0.001, n = 116). Fetal log plasma glucagon concentrations at 26 and 28 days gestation were negatively correlated with body weight, but were not significantly related at 30 days gestation. Neither glucose nor glucocorticoid concentrations were significantly related to bodyweight in individual fetuses. Plasma somatomedin activity in litter-reduced fetuses at 30 days gestation, was significantly higher (p less than 0.02) than in normal fetuses, but was not significantly correlated with fetal body weight. The observation of strong positive correlation between insulin and bodyweight throughout the last third of gestation reinforces the belief that fetal insulin plays an important role in the regulation of both placental and fetal growth.