How Oral Contraceptives Work
The estrogen in the combination, triphasic, and extended-release pills prevents conception primarily by inhibiting ovulation. The progestin in these pills provides secondary contraceptive protection by thickening and chemically altering the cervical mucus so that the passage of sperm into the uterus is hampered. Progestin also causes changes in the lining of the uterus, making it less receptive to implantation by a fertilized egg (Larimore & Stanford, 2000). In addition, progestin can inhibit ovulation. The progestin-only pill works somewhat differently. Most women who take the progestin-only pill probably continue to ovulate at least occasionally. The primary effect of this pill is to alter the cervical mucus to a thick and tacky consistency that effectively blocks sperm from entering the uterus. As with the combination pill, secondary contraceptive effects are provided by alterations in the uterine lining that make it unreceptive to implantation.