When patients ask me how emergency contraception prevents pregnancy, I tell them that it’s primary mechanism is to delay ovulation (release of an unfertilized egg from the ovary). There is no evidence that the EC aborts or prevents implantation of an already fertilized egg.
The efficacy of EC depends on where you are in your menstrual cycle when you have unprotected sex –
- If you are destined to ovulate in the next 1-5 days, EC will delay the release of the egg from the ovary until the sperm have lost their viability in the reproductive tract. This is how it works.
- If you’ve already ovulated and the egg is still in your reproductive tract, you’re possibly already pregnant by the time you take EC. If so, it’s not going to work.
- If you’re not due to ovulate for >5 days or are 5 days or more past ovulation, then you’re unlikely to have gotten pregnant anyway, and the EC hasn’t done much. (But take it anyway, please, since not every ovulation is predictable)
Now a nicely done study reinforces yet again that delay of ovulation is indeed the mechanism by which this important contraceptive works to prevent pregnancy.
Researchers enrolled 450 women presenting for emergency contraception at a clinic in Chile,where they assessed where in their cycle these women were at the time of unprotected intercourse by using hormone assays and ultrasounds throughout the rest of that menstrual cycle to see if they ovulated, menstruated or became pregnant. (Previous studies relied on menstrual history alone to pinpoint ovulation, a much less reliable methodology.) The EC used in this study was levonorgestrel.
Altogether, 103 women who took EC did so just prior to ovulating – Although 16 pregnancies would be expected in this group based on normative data, none of theses women became pregnant. In contrast, 45 women took EC on the day of ovulation – 8.7 pregnancies would be expected to occur in this group , and in fact, 8 pregnancies did occur.
The efficacy when used before ovulation was 100%. On the contrary, when used after ovulation has occurred, the number of observed and expected pregnancies is not statistically different, indicating that no reproductive process subsequent to ovulation is interfered with by LNG-EC. This finding is incompatible with the inhibition of implantation by LNG-EC and is consistent with the mechanism of action of EC reported in a recent review.
Other research has shown that EC does not alter the proteins in the endometrium necessary for the implantation of the fertilized egg.
Although overall, EC has not had a major impact on unplanned pregnancy rates in the United States, it remains an important method of contraception for women. It’s important to counteract misinformation about its mechanism of action for women considering it’s use.
More info on EC from The Emergency Contraception Website