Unprotected sex happens. It can happen when a contraceptive method was not used or not properly used or when the method fails such as a condom breaking or slipping off.
Sexual assault can also cause an unwanted pregnancy. Emergency contraception (EC) is a woman’s last chance to prevent pregnancy in such cases.
Fortunately, in these cases, pregnancy doesn't immediately happen after sex.
For starters, sperm has the ability to survive in a woman’s body for five days after sex. Fertilization (the union of an egg and sperm) must happen within 12-24 hours of ovulation. And implantation doesn’t happen until 7-14 days later.
In a nutshell, emergency contraception works by stalling an ovary from releasing an egg. If the sperm and egg can’t join, pregnancy is impossible.
There are two kinds of EC: emergency contraception pills (ECPs) and the Copper-T IUD.
One type of ECP is levonorgestrel (brand names: Plan B and Next Choice). Levonorgestrel is most effective when taken as soon as possible after intercourse. However it can still reduce the risk of pregnancy when taken up to 120 hours (five days) after intercourse.
During a typical menstrual cycle, an intricate interaction of hormones prepares the body for pregnancy.
The release of luteinizing hormone (LH) over a 24-48 hour period is a critical part of this process. The LH surge causes the ovary to release an egg. ECPs interrupt that interaction, thus preventing or delaying the egg’s release.
The second ECP, ulipristal acetate (brand name: ella), is a different type of medication. It delays ovulation and may help prevent implantation. Ulipristal acetate remains effective up to five days after unprotected sex.
While levonorgestrel pills are effective only if taken before the LH surge, that’s not the case with ulipristal acetate. Ulipristal acetate can still work when the LH surge has begun.
This is good for women who had unprotected sex just before ovulating – when the LH surge has begun – and are at the highest risk of pregnancy.
The most effective form of EC is the Copper-T IUD. A doctor can insert it into the uterus up to five days after unprotected sex. Furthermore, left in place, it can provide at least 10 years of contraceptive protection.
The Copper-T IUD doesn’t affect ovulation, but it can prevent sperm from fertilizing an egg. It may also prevent implantation of a fertilized egg, according to the Office of Population Research at Princeton University.
Princeton University added that there is no point in a woman's menstrual cycle when ECPs (available in the United States) would end a pregnancy once it has begun. They have no effect on women who are already pregnant.
"Emergency Contraception: How Emergency Contraceptives (the Morning after Pill) Prevent Pregnancy." Emergency Contraception: How Emergency Contraceptives (the Morning after Pill) Prevent Pregnancy. Web. 10 Dec. 2014.
Gavin, Mary. "Emergency Contraception." KidsHealth - the Web's Most Visited Site about Children's Health. The Nemours Foundation, 1 July 2013. Web. 10 Dec. 2014.
"The Morning-After Pill - Emergency Contraception - Cost & Info." The Morning-After Pill - Emergency Contraception - Cost & Info. Web. 10 Dec. 2014.
Trussell, James, and Kelly Cleland. "Emergency Contraception: How It Works (How It Doesn't)." ScienceFriday.com. Web. 10 Dec. 2014.
Reviewed December 11, 2014
by Michele Blacksberg RN
Edited by Jody Smith