Emergency contraception is used to prevent pregnancy after unprotected intercourse.
It is less effective in preventing pregnancy than pre-coital methods of birth control, and, while the methods are safe, the side effects can be unpleasant. When high doses of birth control pills are used for emergency contraception, fertilization of the egg is inhibited in two ways. It either prevents or delays ovulation, or alters the transport of the sperm or egg down the fallopian tube. The oral hormones also alter the inner lining of the uterus, making a poor environment for implantation of a fertilized egg. The exact effect of an IUD on preventing pregnancy is unknown, but it is thought to interfere with implantation in the uterus.