I don't get it. Fertility Friend tells me that I'm fertile for a couple of days before and a couple of days after I ovulate (up to three days before and after, to be precise). The Miriam Stoppard book I'm reading says that I'm fertile for about a day before ovulation, and possibly a day after. Baby Centre says that I'm fertile 1-2 days before ovulation, but there's some doubt about whether or not I'm fertile after. Some sites say that sperm can live (and thus fertilize an egg) for up to 5 days. Dr. Stoppard says it's more like 24 hours.
Why is this such an inexact science? Modern medicine, my eye.