On a normal lifespan, it takes a baby around 3 days to grow up in The Sims 3. After this point, the baby becomes a toddler and requires more attention in the game.
Since babies can't move around and perform actions in the game, as is in real life, taking care of a baby in The Sims 3 requires more effort. They require diaper changing time, which brings the hygiene bar to full, and feeding; however, this is moderate compared to the time required for a toddler or an adult.
The easiest way to care for a baby is to make sure all their needs are satisfied before he or she goes to bed. Placing a teddy bear in the baby's inventory is one way to quickly raise the child's mood.