Thus HCG prevents the deterioration of the corpus luteum at the end of the fourth week and enables pregnancy to continue beyond the end of the normal menstrual cycle. As pregnancy continues, the placenta becomes a major source of progesterone , and its presence is essential to maintain pregnancy. Birth Toward the end of pregnancy, Secretion of estrogen by the placenta rises . This is converted into estrogen by the placenta. The rising level of estrogen causes the smooth muscle cells of the uterus to synthesize connexins and form gap junctions .
Gap junctions connect the cells electrically so that they contract together as labor begins.
A number of prostaglandins also appear in the mother's blood as well as in the amniotic fluid. Both oxytocin and prostaglandins cause the uterus to contract and labor begins. Three or four days after the baby is born, the breasts begin to secrete milk. Milk contains an inhibitory peptide . If the breasts are not fully emptied, the peptide accumulates and inhibits milk production.