Kids don't just randomly pop out (or at least, I hope not). You need to be with your wife (preferably her in your party), so you can, you know, do the deed. That's how it works IRL, lol.
As for the game, I've assigned her as the governor of a town. Travel to the town and "wait there for some time". Spend a day and take off. I usually will get a message within a day or two that she is preggers.