If you flirt with someone a few times (you don't need to have a proper relationship yet) then lay on the bed. Invite them to cuddle, then woohoo. Get off the bed and kiss them. Then try 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 mum, you can always leave the other sim if a relationship has started by this point, or you can start a proper family. Use the cheats already on this site to determine wether the baby is a boy, girl, twins triplets etc.