Before OFB, there were rumours that eating cookies or spaghetti would increase a sim's chance of twins. There was also a fake cheat "twinsr2cute" which didn't do anything but the rumours swept all the sims 2 forums. Everyone was SO excited by twins.

With OFB they added the forcetwins cheat and the cheesecake. I think the in-game chance of twins might have been increased, too. As to why it's cheesecake? Not sure - maybe it was an in-joke at the studio, or maybe it's to do with pregnant women craving cheesecake? I don't think it has that association outside of the sims universe.

And yes it's always been a 100% chance. The fix is better because I would always just make them avoid cheesecake just in case. Now I can let them eat it if they want to.

Actually, is there any mod which reduces the natural chance of twins? I like using the triplets and quads mod and in some testing recently I discovered that if a sim is naturally (or cheatedly) pregnant with twins, triplets and quads won't give them a single birth unless you choose the "pick > single" option. Which means that, effectively, I have my triplets and quads odds for multiples, plus the natural chance of twins on top of that. I'll probably just reduce the trips & quads setting for twin chance to 0% (so I only get the natural twins) but it would be cool to be able to nitpick this more exactly.

There's a video here about twins and the infamous broken pregnancy/"twin" token (plus links to some fixes), and a few other things related to twin births/pregnancy: https://www.youtube.com/watch?v=VQBGidaf35g

Almost forgot you could also increase the chance of having twins with the fertility booster in the aspiration reward system (family sims). Perhaps not a good idea to do it for both sims if you want a chance of having single births...

I'm confused. Doesn't Triplets and Quads overwrite the normal choosing of baby numbers?

Pretty sure it does, at least for the "random" option. You shouldn't get the original 10% on top of whatever the T&Q mod is set to. I don't know if there is a "no single baby if pregnant with twins" bug there, but could be (maybe based on the pregnancy controller already being set to twins?).

Twin chance ingame is 10%, which is in effect if you choose "original" (if it's originally twins, you get twins. If it's single, you get single). When using cheesecake or Forcetwins, you'll get twins.

"Pick" should give you the number picked, regardless of chance or original number.