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. This autocrine action thus matches supply with demand. So whether it has an important role to play in human births remains to be seen. Activins , Inhibins , Follistatin . These proteins are synthesized within the follicle.