Instructions
Save your game during the time when your Sim is pregnant but before she begins to go into labor. The best method is to save frequently during the Sim's pregnancy. When it comes time to give birth to the baby, a small cutscene will play and the baby will be born instantly, so be sure to save before this occurs.
Exit to the neighborhood without saving your game if your Sim gives birth to a baby boy.
Load your previous save game state and wait until your Sim goes into labor again. Each time your Sim gives birth there is a 50 percent chance of having a girl so you may have to repeat this strategy several times until your Sim has one.
Save your game immediately after your Sim has a baby girl.