Prostaglandins are neither protein or steroid hormones, but chemical mediators, or "local" hormones. Whereas hormones circulate in the blood to influence distant tissues, prostaglandins act locally on adjacent cells.
The fetal membranes of the amniotic sac, the amnion and chorion, are both believed to be involved in the production of prostaglandins. Various maternal and fetal tissues, as well as the amniotic fluid itself, are considered to be sources of prostaglandins, but details about their composition and sources are not well defined.
It is widely believed that prostaglandins play a part in softening the cervix, initiating labor and/or maintaining labor, but the exact mechanism is unclear. As a result, prostaglandin inhibitors (aspirin and indomethacin) are sometimes used to treat preterm labor.