It can. What Plan B does, is it prevents ovulation. But it also makes your uterus a hostile environment if you have already ovulated. Since it is a massive dose of hormones, it can cause your period to be delayed.

my bf wants me to take plan b because we had unprotected anal and im not on the pill. (im supposed to start nuvaring when i get my period, which was supposed to be 2 days ago)
i wouldnt let him finish without a condom, so he put one on and then finished in my vagina. durring the anal his penis slipped down and touched my vagina once or twice, but no penetration or anything. idk im worried that i havent gotten my period yet, but i dont know about taking the plan b. i think it would make me really sick, because i couldnt handle the pill. it made me sick super to my stomach after 2 days. what do you think i should do?

It does not cause the uterus to be a hostile environment. It actually does the opposite. Plan B is the hormone progesterone , which is the same hormone fertility clinics give women to maintain the uterine lining for keeping a pregnancy.

Here is exactly how it works:

The ovary contains a supply of eggs. One egg each month begins to ripen and mature into a large structure called a follicle, regulated by a hormone from the pituitary called Follicle Stimulating Hormone, FSH. The follicle is going to produce estrogen, which is what causes the growth of the uterine lining. FSH is particularly important early in the cycle.

A second pituitary hormone, Luteinizing Hormone, (LH) assists in stimulating the mature follicle, and in particular, a rapid surge in LH is what triggers the follicle to pop, releasing the egg, in a process called ovulation. This is a key step, so remember this: a spike in LH levels triggers ovulation.

After ovulation, the egg goes on its way, and might be fertilized, leading to pregnancy. The follicle left behind isn't done yet, though; it becomes a structure called the corpus luteum, which continues to produce estrogen (necessary to maintain the uterine lining), and also gradually produces more and more progesterone. Progesterone helps maintain the uterine lining, (This is why women undergoing fertility treatment are sometimes given progesterone to help maintain the pregnancy}. but also suppresses LH production by the pituitary. It's a kind of timer. The corpus luteum is maintained by the levels of LH, but the corpus luteum also produces rising levels of progesterone, which shut off LH…and when too little LH is produced, the corpus luteum shuts down, no estrogen and progesterone are produced, the uterine lining is no longer maintained, and a woman finds a bloody mess in her panties.

The key thing to remember about this part, though, is that progesterone suppresses LH.

Put two pieces of the story together: a spike in LH levels triggers ovulation and progesterone suppresses LH. Hmmm. This suggests an idea. If you wanted to prevent ovulation, how would you do it?

(Consider this a test. Imagine that Jeopardy jingle playing right now.)

Time's up—I bet everyone came up with the right answer, though. Giving someone a large dose of progesterone would shut down LH production, so there would be no ovulation, so no egg would be released, and any sperm happening to be in the woman's reproductive tract would find nothing to fertilize.

You have just figured out what is called Plan B contraception. It is a form of birth control that tells the woman's ovaries to hold off on releasing any eggs for a short while. It's called emergency contraception, because it is used by a woman who has, for whatever reason (rape, a broken condom, misplaced enthusiasm, second thoughts, anything) had unwanted sperm in her reproductive tract, and she wants to make sure that this isn't the moment her ovaries happen to pop a follicle.

First of all, Plan B only works up to 5 days after unprotected sex, so how long ago did this occur?

If the pills that made you sick were progesterone-only birth control pills, Plan B may react the same way with your system, since Plan B is the exact same thing, only a higher dose. I was the same way on the pill as you, I couldn't stand the nausea, etc. However, Plan B will only make you sick briefly (if it's going to make you sick), pregnancy will make you sick for weeks and weeks. At least that is what happened in my case., when I was pregnant.

Futureshock, please explain how it prevents an egg from attaching to the uterin lining. BECAUSE I'M FASCINATED TO KNOW!

How much more explicit could I possibly be? I just posted the exact biology of Plan B.

If biology doesn't work for you, how about pro-life websites?


Conclusion

Given the above, there is no evidence that shows that the endometrial changes produced by COCs contribute to failure of implantation of conceptions, nor is there evidence that COCs cause an increased per pregnancy ratio of ectopics.

There is no evidence that Plan B affects implantation.
# There is evidence that suggests that Plan B only prevents conception by either making cervical mucus hostile to sperm or preventing ovulation.

"'The post-fertilization effect was purely a speculation that became truth by repetition,' says Joe DeCook, MD, a retired OB/GYN and vice president of the American Association of Pro-Life Obstetricians and Gynecologists."

It doesn't prevent a fertilized egg from attaching to the uterine lining.

Plan B is progesterone.
Progesterone helps maintain the uterine lining, (This is why women undergoing fertility treatment are sometimes given progesterone to help maintain the pregnancy}.
Progesterone stops ovulation because it suppresses LH production by the pituitary.

I can tell you where the mistaken notion started, and that was from looking at women on the pill. If there is no ovulation, there is no cascade of events leading to thickening of the uterine lining so implantation can take place. That's the "hostile environment". However, you can only have an insufficient uterine lining of there is no ovulation. The notion of ovulation occurring while a woman was on the pill, getting fertilized, and not implanting was based on the notion that women on the pill have an insuficient uterine lining. What was not taken onto consideration was the fact that if ovulation occurred while on the pill, the cascade of events leading to a thickening, sufficient for pregnancy uterine lining would also occur.

Otherwise, you'd never hear of a woman giving birth after getting pregnant while on the pill. If Plan B stopped fertilized eggs from implanting, it would work a lot longer than 5 days after sex, and it would work a lot better.

"However, you can only have an insufficient uterine lining if there is no ovulation."

Just wanted to pop in and say, this is not necessarily true. Insufficient lining can be a cause of early miscarriage.