A Few Facts For Everyone:
1. Yes, sperm can live for up to 72 hours.
2. The Egg only lives for 12-24 hours.
3. The female ovulates on approximately the 14th day of her cycle, the first day starting on the FIRST day of her period.
4. Birth Control Pills can affect a woman's cycle (obviously), and can contribute to a woman becoming pregnant on her period(If the pills aren't used correct or if a woman stops using them, it could take a while for the body to get back into a normal routine).
So with these facts stated, let's think a moment... if a woman and a man have sex, let's say, on the last day of her period. And his sperm lives inside her for 3 days..that puts a lot of us on the 8-10th day of our cycle. And most females ovulate on approximately the 14th day, but let's say for whatever reason her body decides to ovulate a few days early. Well... then to me... it is possible to get a woman pregnant on her period, but highly unlikely. Of course the improper use of(or when a woman stops taking) Birth Control Pills can throw off a females hormones, and cause her to ovulate at odd times, increasing the risk of becoming pregnant during her period.
Every female is different. Everyone ovulates at different times and for different lengths of time. NOT approximately the 14th day of her cycle cycle.... that's a little naive to make that assumption. A nice way to know when you are ovulating is to just "look below". If you're beginning to have more cervical fluid down there, then you are beginning to ovulate or just finishing. The days where there's more wet fluid, it is assumed to be the day ovulation takes place.