If you have room in the house to spare, The Sims 3 can help you quickly occupy the empty space with twins and triplets -- or even more. There is no way to guarantee multiple births, especially since no fertility cheat code exists, but there are ways to heavily influence your Sim's chances.
Increasing Fertility Chances Naturally
Unlocking the Fertility Treatment lifetime reward is the most direct way to influence your Sim's chances of giving birth to multiple babies. At a costly 10,000 points, this is a reward you'll need to work very hard for. Be sure to fulfill every wish and whim your Sim desires to give her reward points as quickly as you can earn them.
If you don't mind using cheats, you have other options available to you. Enabling testing cheats from the main menu lets you manipulate the amount of lifetime rewards you have. Alternatively, you can enter Create-a-Sim and change your Sim's lifetime wish to one of monetary value like Swimming in Cash; use the motherlode cheat code to raise your household money, instantly rewarding you with enough lifetime reward points to purchase Fertility Treatment.
Other Activities to Consider
If you'd rather not cheat or spend all your time completing pricey Sim wishes, you can try a few other activities at home. Once a Sim has gotten pregnant, have her spend most of the day watching the Kids channel on the television. If your Sim prefers listening to the radio, have her listen to children's music instead.
For best results, try mixing up these activities throughout the pregnancy; rather than focus on listening to as much children's music as possible, you can improve your odds a little more from a combination of both.
Influencing the Gender of Your Sims
Some Sims can determine a baby's gender. To do this, a Sim must be employed in the Medicine career path and be at least level five of the career track. To influence gender, try eating apples to have a baby boy or try eating watermelon to have a girl instead.
Preparing the Household for Two
The only thing left to do is prepare for the birth; you'll need to be sure the babies have everything they need when they arrive.
Cribs: Every baby needs a crib, and you'll need at least two if you're expecting twins. You won't be able to put two babies in the same crib.
Toys: During the infancy stage, mobiles over the crib will keep the little ones entertained. When they grow up, they'll be able to play with several toys that double as development tools; for instance, playing with the xylophone gives toddlers a boost toward music skills as they grow. Make sure to purchase multiples of every toy if you're expecting twins or triplets.
High Chairs: Though not used during the infancy stage, you'll need two separate high chairs when the babies grow to be toddlers.