I haven't noticed either a problem having babies or a lack of multiples.

The only oddness I've noticed is that if the game is paused immediately after a baby is released, and then the game is left off for a few hours, when you unpause you can immediately try for a baby again. Whereas, if you tried to have another baby immediately, they will say it's too soon. So pausing and leaving it off seems to bypass the "too soon" thing, which I'm sure wasn't intentional. I had several couples who are "No Way" or "Not Really" all have a new baby when their youngest was still 2 using this method. It doesn't work 100% and they obviously don't always have a baby when they try for one, but most of the time they'll at least try.