Birth Control Pills
Progesterone and estrogen hormones exist naturally in women. Birth control pills contain synthetic forms of these hormones. Some pills only contain progestin, the synthetic form of progesterone according to PlannedParenthood.org. Other pills contain both estrogen and progestin. Because the birth control pill works by impacting hormone levels, missing a pill or taking it at a different time each day will negatively impact the pill's efficacy. Besides the week of menstruation, hormone levels will remain constant due to the birth control pill. Pregnancy prevention with the pill can be as high as 99 percent but only when the pill is taken according to the manufacturer's instructions.
Ovulation
Pregnancy can only occur during ovulation if an egg is fertilized by sperm. According to HealthyWomen.org, ovulation occurs about 14 days into the menstrual cycle. Estrogen levels rise at the start of the menstrual cycle. As estrogen rises, the lining of the uterus thickens to prepare for a fertilized egg. During ovulation, an egg is released by the ovaries.
Birth control pills containing estrogen prevent the levels from fluctuating by keeping them constant until it is time for the uterine lining to shed, according to Mayo Clinic. HealthWomen.org points out that without a peak in estrogen the ovaries never receive a signal to release an egg.
Sperm Blocking
Hormones in birth control pills thicken the cervical mucus to keep sperm from entering the cervix and fertilizing an egg that may have dropped. Once ejaculated, sperm can only survive for up to 72 hours. If they become trapped in the thickened cervical mucus for too long, they will die before having a chance to reach the egg. Birth control pills make the uterine lining unfavorable for the implantation of a fertilized egg. However, PlannedParenthood.org suggests there is no way to prove that this occurs.
