The uterus is different during early pregnancy than it is at term or immediately postpartum. It contains far fewer oxytocin receptor sites - places where oxytocin can be absorbed. The lower ability of the uterus to absorb oxytocin during early pregnancy suggests that oxytocin will not cause effective contractions during that time. For most of pregnancy, progesterone is the dominant hormone. Toward the end of pregnancy estrogen blood levels become higher than progesterone levels in preparation for labor.
Progesterone relaxes smooth muscle cells.
Since the walls of the gastrointestinal tract and veins have a smooth muscle layer, this effect of progesterone can contribute to pregnancy discomforts such as heartburn, varicose veins, and susceptibility to urinary tract infections. At the same time, the high progesterone levels of pregnancy are highly effective at keeping the smooth muscles of the uterus quiet until it is time for labor to begin. In the first few days postpartum, when milk production is just beginning, women have very high levels of hormones related to milk production, which in turn have a strong effect on the uterus. When the baby suckles, those hormones affect the uterus. Over time, as the mother's body becomes accustomed to the stimulation of the suckling infant, much lower hormone levels are needed to maintain lactation.