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Heather Corinna replies:
Your body doesn't really care how long intercourse goes on for.
Whether it's 30 seconds or fifteen minutes (and it's worth mentioning that five minutes of intercourse is about average, so that's not a short period of time for that activity), what your body cares about if it has been exposed to viable sperm, as well as any diseases or infections. And all of those things can, and often do, easily happen within five minutes of intercourse.
What we know about pre-ejaculate is that when sperm is likely to be in it is when a male has not urinated before sexual activity, or after any ejaculate or pre-ejaculate. Given it doesn't sound like that happened, and that you were exposed to pre-ejaculate -- and understanding that pre-ejaculating isn't something that just happens once, but often throughout arousal and/or intercourse, and neither partners will usually feel it during intercourse -- you may have had a low risk of pregnancy. I say low, because given far less sperm are in pre-ejaculate than in full-ejaculate, it does present a lower risk. However, people have become pregnant this way, and if a woman was in a fertile period during that exposure, her risk would be greater.
In general, it is NOT very likely that you were ovulating the day before you expected your period. Rather, if, indeed, it was the day before your period, we can generally agree that you were not in a fertile period.
However, if you really do not wish to become pregnant, we'd still advise emergency contraception for any instance of unprotected intercourse. You can use that within 120 hours (five days) of your risk, but the sooner it is used, the more likely it is to be effective. It might be wise to just go and get it now: by the time you get it, if your period really is due within one day, you may get that period and thus, clearly have no need for the EC. But then you'll also have a pack around should you ever need it again, so it's a win-win situation.
Too, you've also potentially been exposed to any number of STIs. I know that seems like less of a big deal, but it's really not: some STIs can change the course of your life just as much as a pregnancy can, especially for those that go unnoticed and untreated, as well as for the few which are potentially fatal. Statistically, you're actually more likely to wind up with one of those than you are to wind up pregnant. Infections are something you can't prevent after the fact. But you and your partner should schedule a screening for around a month from now, and always be sure to get those every year thereafter. All sexually active people should have those tests yearly, even those of us who do always practice safer sex.
You might know this already, but just in case you don't: withdrawal isn't an effective method of birth control, especially for younger people, and it does nada when it comes to reducing the risks of sexually transmitted infections. So, if you and your partner aren't using condoms, it's time to start. If you're not using them consistently, it's time to change your habits so that you are. Condoms can ONLY be fully effective when they're not only used properly, but used consistently -- as in, every time -- and for all, not just some, genital contact, from start to finish. If the two of you are having a tough time being sexually responsible in this way, it might be time for a talk about taking a break from sex until you can be. If your partner isn't being supportive about using condoms, then you'll need to state and set a clear limit, that from here on out, it's no glove, no love: he needs to use a condom every time, and if he doesn't want to at a given time, then you won't have sex at that given time.
Here are a few more links for your reference: