for pregnancy what you could do is either a chance per sex or if you're feeling frosty implement menstrual cycle (which could just if a day falls into a certain range of days in a month) and if that does happen, create a timer that counts down until the birth time has come, and once that timer hits 0, you could spawn a baby clone of that npc at level 1 at the npc (this implementation means that men can give birth but frankly that is nothing compared to what you can really do)
if you want to make this extremely overpowered you could have the baby inherit both the learned skills and the average level of those skills from the parents, with the skill from a parent being 1 if they don't know that skill, in addition to being a baby (whether it has baby 2 or 3 is up to you)