Sometime on Day 3, the Sim will deliver her baby. The camera will bring your attention to your Sim when she is ready to have the baby. The game will pause and family members will gather to watch the baby enter the world. If you save the game before the baby is born, and you don't get the sex you want, you can restart the game and try again.
A screen will show up saying that a new family member (or two new babies) is on their way. The new baby will be in the Sim's arms. You need to choose a name for the baby. The name cannot be changed, so pick one you like!
Good Luck! Now the real fun of taking care of a baby and toddler will be coming soon. Maybe next time you'll have twins.