You can also influence your child's gender while the mother Sim is pregnant. That's all I know for certain about how to get multiples, though I've also heard that young adult female Sims are more fertile than adult females and therefore more likely to have more than one baby at a time. I tried this twice, and both times she ended up with twin girls. I went back to my last save, about four Sim hours before she was due, and had her watch Kidz Zone while listening to Kids radio. I have a young Sim woman on my game who recently had a single boy, but I suddenly decided I wanted to see if I could get her to have twins. I was actually surprised how well this one works. Other ways to increase chances of multiples are having your Sim listen to the Kids station on their stereo and by watching the Kidz Zone channel on the television *while she is pregnant*. In case you don't know how, this is the way to get the 'Fertility Treatment': On the other hand, I don't know if it has any effect on the number of babies if your Sim is already pregnant when you get it. Having this for both parents helps your chances even more.
To increase the likelihood of multiples, you can buy the 'Fertility Treatment' Lifetime Reward.
