Have a baby, then have someone pick it up. Enable the move_objects on code, and while the person is still holding the baby, go to build mode. Click on the Sim holding the baby and delete that person. Afterwards, the game will say you had a baby. To bring the Sim back, click on that person's icon. You will only have one baby. If this does not work, try again. Note: This may lower your Sim's work skills.