According to About.com, it takes babies about three days to grow out of their infant period in “The Sims.” During this time, the player needs to alternate which partner takes care of the baby every day to avoid one Sim getting low spirits.
It is possible for a Sim to have twins in “The Sims,” but it is not something that was actually built into the game. To trigger this glitch, after a Sim has her first baby, players can make her play the Lovebed a lot with her significant other. Sometimes, this triggers the female Sim to get pregnant again. Although this does trigger another baby being born, both babies are stuck with the same name.