I've had various PCs and noteable NPCs become pregnant in my games. I haven't bothered with ovulation charts etc., instead, if I know characters are having sex I just roll for results if any.
As far as early pregnancy detection, I allow "Detect Life" to pull it if specifically cast for such.
I've yet to see a pregnant PC try adventuring deep into the pregnancy. I have had to make one significant ruling, though: if a pregnant character dies and is revived, the fetus miscarries immediately. Other than that, what happens to the mother happens to the fetus as far as saves etc. are concerned.
Where I *have* had to come up with a chart is for what ends up being born. Gender, number (twins? triplets?), health, etc., all varied somewhat by race. And in the unlikely event a PCs child ends up reaching maturity, I've also dreamed up a way of determining its stats based on those of its parents and some randomizing. (right now, in fact, one player in our new game is playing her old PC's daughter...)
Lanefan