Risk of pregnancy without ECP. The risk of pregnancy from a single act of unprotected midcycle intercourse (at the time of ovulation, about 2 weeks before your next period is due) is about 20-30%. If the intercourse occurs at a different point in the cycle, the risk of pregnancy is about 2-4%. However, it is difficult to estimate the time of ovulation so it may be hard to estimate your risk of getting pregnant.
ECPs contain the same hormones as in some birth control pills (estrogen and progestin) but in different doses and with different timing. The hormones affect the hormone release from the ovary and disturb the development of the uterine lining, thus making an inhospitable environment for fetal implantation. You cannot use regular birth control pills to prevent pregnancy after unprotected intercourse. ECPs are about 75% effective in preventing pregnancy.
Risks and side effects. Risks are similar to those of the Pill, but much more unlikely because of the short exposure. However, ECPs are not recommended for women who suffer from undiagnosed, abnormal vaginal bleeding, impaired liver, kidney, or heart function, or porphyria. Also talk to your health practitioner if you have ever had a stroke, breast cancer, or blood clots in your legs or lungs. The following serious side effects of routine oral contraceptives are rare in healthy young women: blood clots, heart attacks, strokes, liver disorders, gallbladder disease, migraine headaches, and high blood pressure. If you suddenly suffer from abdominal pain, chest pain or shortness of breath, severe leg pain (calf or thigh) and sudden vision change or loss, you should see a health practitioner immediately.
Temporary side effects of routine oral contraceptives include breast tenderness, nausea (it may help to take the pill with food), weight gain or loss, depression, dizziness, and water retention. Any of these may occur with the use of ECPs. The most common side effects of ECPs are nausea, vomiting, and menstrual irregularity after use. These side effects should subside within one or two days of the completion of treatment. ECPs may also alter the timing of your next period, but it should begin within three weeks after treatment.
Failure. Continuation of a pregnancy if ECP fails may pose some slight risk to the fetus but because of the small number of cases, the exact extent of the risk is unknown.
Where to obtain ECPs. Columbia Health Services provides ECPs, but many other health care providers don't. So, if you are out of town or do not use Columbia Health Services, call the new Emergency Contraception Hotline at 1-800-584-9911 to find the nearest provider of ECPs. You can also find a directory of providers and other information at Emergency Contraception.
Sources: Contraceptive Technology by Robert A. Hatcher et. al., 16th ed., "Emergency Contraceptive Pills" (a pamphlet published by Columbia Health Services), and Chapter 21 of The New Our Bodies, Ourselves (1992 ed.).