Do you read the tips as the game loads? One tip I saw recently said "A villagers likes and dislikes affect their behavior"

I had trouble getting my villager to have kids (she wasn't useful for anything else) and I noticed that her only dislike was, of course, children. (I made her to become a master parent anyways lol)

Just keep trying. It's not about science as much as it is about medicine. Try to get Level 2 of medicine asap if you are still having trouble. The chances of having a baby raise when medicine is at level 2 and 3.