Originally Posted by Charbucks

Another thing that should be made clear: whenever you read "X days after menstruation", that means after the START. So if the girl ovulates 9 days after a 5 day period, that's only 4 days after she stops bleeding. However, sperm can live in the woman for up to 5 days, and fertilize an egg at any point during that time, so basically, you don't have a "window". At least, not a guaranteed safe window.

I personally know at least 3 people who were born as a result of their parents using the "natural rhythm" method. Condoms suck, but unwanted pregnancies suck more.