Your Sims must be married and the Love Bed must be unlocked. Pick your female Sim and tell her to "Sleep". Then, bring the male Sim and tell him to "Play In Bed". After you finish, kiss her and you will be asked if you would like to have a baby. Note: Your relationship must be 100%.

If you need to have a baby as part of a goal, in free play mode make a family with just a male (if playing a female) or a female (if playing a male). Then, make them have a child. Save them, then move them in on 3 Free Street. Build a small nice house then go back under your Sim that need to do this goal. Have him or her get in love and then marry. Make sure this is the last goal that you need.. This way, you can just move out and move on with the game. When you move in with the next family and view your goals, "Get married" and "Raise a baby to child" will be already be checked.

Use the following trick to have a baby on a low budget. If you have not unlocked the Vibrating Bed, use this instead. Tell your Sim to call your mate over. Then give them a gift, kiss them, hug them, then kiss them again. You have to do this immediately. That means they must be lined up at the top. You can do this without being married. Your Friendship meter must be high.