J.D.
I highly suggest you get the book Taking Charge of Your Fertility by Dr. Toni Weschler. It is very unlikely a woman would get pregnant a few days prior to their period. The hormones which cause the lining to shed occur after ovulation.
A woman is only able to conceive in a 24 hour period which normally occurs 10 - 16 days after day 1 of her period. The next period normally occurs 24 - 28 days after day 1. Every woman is different and every woman's cycle is different from month to month. The book I recommended helps a woman to know her body and to know her cycles to be able to know when she can achieve/avoid pregnancy.
I hope this helps!