Flirt with someone a couple times (you don't need to have a proper relationship yet) then lay on the bed. Invite them to cuddle and then get busy. Afterward, get off the bed and kiss them before trying for Baby. After this your Sim will start to throw up every so often; don't let them eat bad food. If your sim would rather be a single mom, you can always leave the other sim if a relationship has started by this point. You can also start a proper family. Use the cheats already on this site to determine whether the baby is a boy, girl, twins triplets etc.