Ovulation is day 14 of a womans cycle (average). Day one is from the first day your period starts. So if a woman has a 7 day period, she ovulates 1 week after her period. Just pretend she has a 5 day cycle and has sex with her 3 days later. That would be day 8 and probably not close to ovulation. But pretend she has a 7 day cycle and has sex on day 10 - now we're in a little more gray territory and this is why a DNA test is required to prove anything.

If those two aren't willingly going to do a DNA test, a court might have to get involved to sort out whose kid it is.