What is the efficacy of emergency contraception methods?

Updated: Oct 06, 2020
  • Author: Frances E Casey, MD, MPH; Chief Editor: Richard Scott Lucidi, MD, FACOG  more...
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Placement of a copper IUD within 5 days of unprotected intercourse is the most effective method of emergency contraception, with a failure rate of only 0.09%.  Ulipristal acetate is more effective than levonorgestrel alone, preventing about 2/3 of undesired pregnancies. Levonorgestrel loses effectiveness over time, preventing about 1/2 of undesired pregnancies up to 72 hours. Despite this significant reduction in the rate of pregnancy, patients must understand that this method of contraception should be used only in emergencies and that they should be encouraged to use other more consistent forms of contraception.

Several factors complicate the calculation of a failure rate. Factors include dependence on the patient's history of their last menstrual period and day of exposure, effect of regular and irregular menstrual cycles on the calculation of the estimated time of ovulation, the possibility of the patient being pregnant, and the possibility that more than one unprotected coitus has occurred during that period.

Studies have demonstrated a lower effectiveness for levonorgestrel in women weighing more than 165 pounds. One study has suggested doubling the dose for women with a BMI>30 kg/m2. [37]

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