Coutinho E M, Darzé E
Am J Obstet Gynecol. 1976 Sep 15;126(2):224-5. doi: 10.1016/0002-9378(76)90279-9.
The effect of prostaglandins E2 and F2alpha on the contractility of the uterine cervix was investigated in nonpregnant women throughout the menstrual cycle. PGF2alpha was found stimulatory to the cervix at all phases of the menstrual cycle, whereas PGE2 was inhibitory. The relaxing effect of PGE2 was most marked during midcycle.