Well, I don't remember much about it, but there is a cheat called "boolprop testingcheatsenabled_true" which you can use to get the Tombstone of Life and Death, and impregnate a Sim using any Sim in the household. They are guaranteed to become pregnant. Sorry I can't be of more help with this one; I haven't used boolprop because it gives my computer problems.
Or, you could do it the old-fashioned way: Having a male and female Sim be relaxing together on a bed (or hot tub, or you can use a changing booth) and have one Sim selected, then select the other and pick "Try For Baby". If you hear little chimes at the end, then you're going to have a baby (or babies!) in three days!
BoolProp testingCheatEnabled true The Shift and Click on the mailbox, it will say "Set pregnancy relations with..." you click on that and then click on the name of the father you want.
