Emergency Contraception
The morning after pill, or emergency contraception (EC), is used when a first form of birth control fails or if no birth control method was used. There are different brands, but EC usually contains either just levonorgestrel or a combination of estrogen and progestin, according to Sandhya Pruthi, M.D. from the Mayo Clinic. A high dose of hormones are in the morning after pill, so it should never be used as a primary form of birth control. An similar to regular birth control, EC will not terminate an existing pregnancy.
Ovulation
For a woman to become pregnant, her ovaries need to release an egg, which is fertilized by a man's sperm. However, Dr. Sandhya Pruthi states that "human conception rarely occurs immediately after intercourse. Instead, it occurs as long as several days later, after ovulation. During the time between intercourse and conception, sperm continue to travel through the fallopian tube until the egg appears." Because of the limited window between copulation and fertilization, the morning after pill needs to be taken within 120 hours of unprotected sex, according to Planned Parenthood; the morning after pill is most effective when taken within 24 hours of unprotected sex. The influx of synthetic hormones prevents the ovaries from releasing the egg, thus stopping an unwanted pregnancy.
Cervical Mucus
Besides preventing ovulation, the morning after pill also thickens the cervical mucus, according to Dr. Sandhya Pruthi. The cervical mucus acts as a barrier between the sperm and the egg. When a woman takes the morning after pill, the increased amount of estrogen and progestin or levonorgestrel thickens the mucus in the cervix. If ovulation has occurred when the woman had unprotected sex, the thickened cervical mucus will prevent the sperm from fertilizing the egg.
Uterine Lining
A third way the morning after pill works, according to the Planned Parenthood, is not definitive. Should an egg be released and fertilized during unprotected sex, the morning after pill would prevent the pregnancy by thinning the uterine lining. With the lining of the uterus thinned, it prevents the fertilized egg from implanting in the uterus. As noted by Planned Parenthood, not enough evidence on the morning after pill's effect on the uterine lining is available to make an authoritative decision.
