Monday - last edited Monday
@daughtermitchell I've moved your post to the Sims 3 PC tech section.
There's an easy way to increase your sims' chances of having multiples (twins or triplets, you don't get to choose). Have the mother listen to kids' music, watch the kids' TV channel, and read a pregnancy book or two. It doesn't take all that much effort: when I want twins, I have the portable boom box follow the mom around for the first 24 hours or pregnancy (minus when she's asleep), just as she goes about her business, and that seems to give about an 80% chance of twins, with a small chance of triplets. I think that after 48 hours, the interventions might not have an effect anymore, but I'm not totally sure.
You could also buy one of the parents the Fertility Treatment lifetime reward, but it can't be revoked if you later want a single child, and it kind of seems like a waste of the points when there are easier ways to get multiples.
Without mods, if you don't get the number of kids you want, you'll need to replay the last day or two over and over to try again. With mods, specifically NRaas MasterController, you can "reroll" the pregnancy outcome all the way up to birth with a few clicks, so if you get a singleton birth, you can quit without saving, go back to when the sim was about to go into labor, and try again.
Without mods, I doubt there's any way to make a sim give birth early. With NRaas Retuner, I think you can set the overall length of a pregnancy to whatever you want; the default is 72 hours. NRaas DebugEnabler lets you set the length of a pregnancy for a specific sim, and it also has a Have Baby command. If you're interested in the specifics, search for "Sims 3 NRaas Pregnancy," and you'll get plenty of information.