How common are twins?

Not that common but the easiest way would be to use the Rewards Store and buy a fertility potion.

It costs about 3k points and they are not that hard to obtain.

Not sure, but my first birth was boy twins. I did have my mother sit in front of the tv and watch the kids channel to see if it would increase the chance. I think she sat there a few times, overall perhaps 3-5 sim hours. Not a lot and unsure if it that is why, but it could have helped. ;) She was BORED, though. LOL! I'd be interested to see what others have to say about this.

My Sim's second pregnancy was twins. It's the only set I've had thus far. Edit, I've had three Sims give birth:

Juliette - Single, twins, single, single, single
Toula - Single, Single
Nina Caliente - Single, Single

I've only had three pregnancies and no twins yet. I'll have to see how it goes for the next generation once they get to that stage of life.

I've been hearing that it's more common to have twins than before. My sim had twin boys.

My sim is currently pregnant with #3! Here's to hoping it turns out to be #3 and #4!!

I have only had two pregnancies in game so far and the second one was twins. I read one here that someone had triplets!!!

I've been getting twins more frequently than in TS3. In fact, the second pregnancy I played through, I got twins. It seems a lot more common than in previous Sims games.

I've yet to have triplets, though :p

Seems like a lot of people had twins the second time. I did as well. My second generation mother had twins, two girls.

My sims always have twins and its actually getting annoying lol. It will be singleton, twins, twins, singleton.. this is where I miss toddlers because it's annoying having 6 kids at once lol