I googled this...
If you have a 28 day cycle, you will generally ovulate on day 14. If you have a 35 day cycle, you will ovulate on day 21. In other words, the first part of the cycle (prior to ovulation) is where the variation is. You will, as a rule, begin your period 14 days after you ovulate, if the egg is not fertilized.
Sperm live 72 hours (the best swimmers usually give up at about 48 hours, however) and the egg about 48.
So, based on a 28 day cycle, if you have intercourse on day 12, and ovulate on day 14, you can get pregnant. If you ovulate on day 14, have intercourse on day 16, you can get pregnant. If you are lucky, and the sperm are REALLY good swimmers, you could get pregnant as early as day 11, all the way through day 16-17.
So that gives you about 5-6 days/month you might find yourself fertile.
Hope it helps.
at 11:53 PM on Jun. 20, 2009