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Will Plan B Work After Ovulation?

I'm a writer and, I must confess, hypochondriac. I am medical-office assistant certified and used to work as a certified pharmacy tech.


First, a Little Lesson in Biology

Will Plan B work after ovulation? This is a very good question considering that the package insert in Plan B mentions this drug being quite effective in delaying ovulation. The fact that this drug is capable of delaying ovulation is reassuring, but only if you are taking Plan B prior to ovulation! What happens if you had an oops moment and it happened right after ovulation?

Let's first take a little step back first and take a closer look into the cycle, when ovulation occurs and which are a woman's most fertile days. Then, we can grasp a better understanding on how Plan B works and whether it can prevent pregnancy even if taken after ovulation.

Your menstrual cycle on average lasts anywhere between 28 and 30 days. These are basically the days in between the first day of your period up until your next period. A menstrual cycle comprises four different phases: the menstrual phase, the follicular phase, the ovulation phase and the luteal phase.

When your period takes place during the menstrual phase, it's basically your uterus shedding its thick lining. An average period lasts roughly 4 to 7 days. The uterus lining sheds only when you are not pregnant. If you are pregnant your uterus needs its thick lining (endometrium) because the wall of the uterus is needed for pregnancy.

In the meanwhile, as you have your period, the uterus doesn't waste time and already starts prepping in the future case a potential pregnancy takes place. Once your period is over, the follicular phase takes place.

The follicular phase generally lasts around a week. During this time, follicles in the ovary mature. Follicles are basically small sacs filled with fluid located inside the ovaries. They contain immature eggs which develop and grow in size during this time.

Next, comes the ovulation phase. In an average 28- day cycle, ovulation happens around day 14. During this time, a mature egg will be released from the ovary all the way down from the fallopian tube where it will be ready to be fertilized. This egg has a very short lifespan, living only about 12 to 24 hours.

The luteal phase concludes the menstrual cycle. This phase lasts from right after ovulation until the next period starts. So whole cycle therefore starts all over.

So what days are the most fertile? The most fertile days in women are the six days before ovulation as well as the day of ovulation. Sperm has a lifespan of 5 days and therefore has a chance of fertilizing the egg when it is most viable during the ovulation phase.

To be precise, out of the six days, the three days leading up to and including ovulation day are the days claimed to be the most fertile. If you have a typical 28-day cycle with ovulation taking place on day 14, your most most fertile days may therefore be days 12, 13, and 14. According to Fertility Coalition, by having intercouse during these days, you have a 27 to 33 percent chance of getting pregnant.


How Does Plan B Work?

Plan B One Step is an emergency contraceptive (meant to prevent pregnancy) also known as "the morning after pill." It consists of one tablet containing 1.5 mg of levonorgestrel, a hormone that has been used in birth control pills for many years.

Unlike the pill though, Plan B contains a higher dose of levonorgestrel which makes it effective in preventing pregnancy after unprotected intercourse when taken within 72 hours (three days). The earlier the pill is taken the better.

How does Plan B exactly work? According to the product insert, plan B works by stopping the release of an egg from the ovary. In other words it prevents the onset of ovulation.

The product insert further adds that on top of stopping the release of an egg from the ovary, Plan B "may also work by preventing fertilization of an egg (the uniting of sperm the egg) or by preventing attachment (implantation) to the uterus (womb)."

Plan B is claimed to be very effective. The product insert indeed predicts that, when taken correctly, about 7 out of 8 women who would have otherwise gotten pregnant won't get pregnant when using Plan B.

The only sure sign of Plan B working is the arrival of the period. Many women though will likely be very anxious before then. On top of that, it doesn't help that Plan B may cause the whole cycle to get out of whack due to the effect of hormones. The product insert indeed warns that, after taking Plan B, the next period may arrive at the expected time, or within a week of the expected time.

If the period is delayed for over a week, then Plan B suggests having a pregnancy test done and following up with a medical professional.

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Will Plan B Work After Ovulation?

If Plan B works in preventing ovulation, then the next question is "what happens if I take Plan B if ovulation has already occurred? In other words, in a 28-day cycle, what happens if unprotected intercourse takes after day 14?

This is where things get murky. According to the package insert Plan B "may also work by preventing fertilization of an egg (the uniting of sperm and the egg) or by preventing attachment (implantation) to the uterus (womb)." Yet the wording "may" provides little reassurance as it implies a certain level of uncertainty.

According to Dr. J, a board-certified OB/GYN: "Plan B works by preventing ovulation. It might also prevent implantation, but the primary mechanism of action is by preventing ovulation in the first place. This is why it's only effective about 90% of the time (because if you take it after ovulation has already occurred, it is not effective)."

Yet how can one know the exact hour/minute an egg is released and ovulation occurs? Dr. J further suggests potentially having an IUD placed up to 5 days after intercourse to prevent implantation if there are concerns about Plan B not being effective because of having unprotected intercourse after ovulation.

On the other hand, Dr. Muneeb Ali claims: "Preventing ovulation is only one of the ways plan B prevents a pregnancy, it has other mechanisms of action, even if it was taken 2 days before ovulation, it would be just as effective. Also, it has been known to prevent pregnancy even after ovulation, so that just goes to show that plan B is a VERY effective preventive measure against pregnancy."

So what's the verdict? It appears that Plan B is most effective prior to ovulation, but after ovulation things get iffy and even experts can't seem to come to an agreement on its efficacy.

So what's next? Perhaps it may turn helpful getting a clearer picture by looking at some studies conducted on this topic.

Plan B causes Progesterone Withdrawal - so hence this causes any embryo NOT to be able to implant. This is why it is important to get this pill in your body before 72 to 96 hours after. So, it does NOT matter if before or after ovulation.

— Dr. Owen

What Studies Say

Many times, we expect studies to help confirm or rule out hypothesis, but many times the results can be inconclusive and disappointing.

According to an article published on Contraception: "Studies have shown that levonorgestrel (the hormone in Plan B) has no postfertilization effect on rats and monkeys (Croxatto et al. 2003; Müller et al. 2003; Ortiz et al. 2004), but the extent to which this finding can be extrapolated to human beings is unknown."

In other words, things are quite murky even along the field of studies...


Finally, What Are the Chances of Getting Pregnant After Ovulation?

One last thing to consider is whether it's easy or not to get pregnant after ovulation. What are really the odds? If Plan B may not be effective as hoped after ovulation, what are the odds in your favor?

We have seen that the most fertile days in women are the five days before ovulation and then that one day of ovulation.

The days following ovulation, which technically fall under the "luteal phase" are not very popular for those who are trying to conceive. Indeed, once the egg has been released, chances of conceiving are low.

If we are looking for percentages, here is a rough breakdown:

  • Chances of getting pregnant are 21 to 35 percent one day before ovulation
  • Chances of getting pregnant are 10 to 33 percent on ovulation day
  • Chances of getting pregnant are 0 to 11 percent one day past ovulation
  • Chances of getting pregnant are 0 to 9 percent two days past ovulation

Now the difficult part is determining when ovulation takes place! When we mention ovulation occurring on day 14 of an average 28 cycle, we are just mentioning an example, but many women have cycles that can be longer or shorter.

Ovulation may therefore generally take place anywhere between the 10th and the 17th day of the cycle. So unless you have an ovulation predictor kit, you may not know when ovulation took place.

Concluding, whether Plan B works or not after ovulation remains a subject of debate even among professionals. Studies appear quite inconclusive. The only way to determine whether Plan B worked or not after ovulation is therefore to simply get a pregnancy test (or several!) done when your period is late.


  • Plan B label information" (PDF). Food and Drug Administration. August 24, 2006.
  • Wilcox AJ1, Weinberg CR, Baird DD. “Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby.” N Engl J Med. 1995 Dec 7;333(23)
  • Postcoital treatment with levonorgestrel does not disrupt postfertilization events in the rat. Müller AL, et al. Contraception. 2003

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This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

© 2018 Alex Ferris

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