Instructions
Begin a romantic relationship with another Sim of the opposite sex. This doesn't have to be in multiplayer mode, you can begin a relationship with a computer Sim. Simply approach a Sim in the game and select the "Talk" option. From there you can select other options such as "Flirt," "Tickle," or "Dance," in order to raise your romance points.
Continue seeing your Sim's romantic interest periodically, until your points are high enough that they officially become boyfriend and girlfriend.
Go to your bedroom and your Sim partner will follow. Select the "Relax" option and they will both get into bed to relax. If the relationship points are strong enough, your two Sims will try for a baby. Otherwise you may have to try a few times. If the female Sim does get pregnant, you will hear the "Rock a Bye Baby" song. Four or five days later the female Sim will give birth.