I DO have mods and play on a PC, but I cannot get my sim pregnant either. I just started the 100 baby challenge and it's been 3 weeks and woo-hoo COUNTLESS times with 4 different sims, and its a no go. The first sims I tried to get pregnant with was Don Lothorio, so I thought it was because he kept using a condom (I have Wonderful Whims installed, hence birth control is possible). But after 3 more Sims, and her sneaking around to not get caught, she is seeing 3 others too. Nothing. It's pretty hard to have a 100 babies challenge if she can't get pregnant.
I am going back into game to make another sim, this time married, and see if I have any luck.
Yes, I have the latest mods, I have been carefully trying to grab them as they are released and believe I have any that would be affecting pregnancy updated. Now, furniture or clothes will be a different story. But not of my mod creators have release any of these, so I don't believe it is a build/buy CC. But if my next attempt to get another sim pregnant fails, I will create a whole new Sims 4 folder and see what happens in a totally unmodded game.