Figure 10.Shown is a representation of the hypothalamic-growth hormone-IGF-I axis, with details of its modification during pregnancy. A. In the non-pregnant state, pituitary GH secretion is regulated through hypothalamic control. Pituitary GH regulates the secretion of IGF-I, which, in turn, exerts negative feedback action on GH at the hypothalamic-pituitary level. B. During the latter half of pregnancy, the GH-IGF axis is inhibited by large amounts of estrogen. The large increase in placental GH exerts an inhibitory effect on GH secretion mediated by placental GH on the hypothalamus and pituitary. (From (133) , with permission)