It's not possible to get pregnant 4 days before your period, but it is possible to get pregnant 4 days before you thought your period was going to start if you ovulate later than expected in your menstrual cycle. There are only a few days during a month when it's possible to get pregnant, and they're grouped around the time of ovulation. Sperm only live 3 to possibly 4 days, and eggs only survive 12 to 24 hours after ovulation, so the window of opportunity for pregnancy is short.
Ovulation is often assumed to take place on day 14 of the menstrual cycle; this is only true if you have 28-day cycles each and every month. The time between ovulation and the start of the next period is fairly fixed at 12 to 16 days, according to the American Pregnancy Association. If your period is prolonged one month, which can happen even to women who normally have regular cycles who are sick or under stress, you may not ovulate until day 24. If this happens, you can get pregnant 4 days before you would expect to get your period, if your normal cycles are 28 days long.
The key to getting pregnant is knowing when you're ovulating. If you know you're ovulating late in a given month, you can get pregnant, even if you ovulate as late as day 24 of your menstrual cycle. Physical signs of impending ovulation include thin, stretchy cervical mucus, Dr. Aniruddha Malpani, M.D. and Dr. Anjali Malpani, M.D. explain. Ovulation predictor kits test the urine level of luteinizing hormone, which rises right before ovulation. If the ovulation predictor kit and your body's signals indicate you're ovulating on day 24, you may get pregnant 4 days before you thought your period was supposed to start.
Knowing you're ovulating as late as 4 days before your normally expected period means that you have to keep having sex until you do ovulate to get pregnant. When it comes to timing sex to get pregnant, people often think more is better, but the truth is that less is more. Having sex every other day rather than every day help builds up a better supply of sperm. Since sperm live several days, having sex every other day starting by day 10 of your cycle ensures that you'll hit either the day before ovulation or the day of ovulation, Professor William Ledger of the National Health System states.