Set one of your female Sim's traits to Kleptomaniac.

Go to the graveyard, and have the Sim steal male remains.

On the second day after putting the male remains in your home, a ghost will appear at 11:00 p.m. and disappear at 4:00 a.m. It will appear every other day.

Socialize with the ghost every night it appears. When you become best friends with the ghost, it will begin appearing every night.

Get the "Fertility" reward.

Confess your attraction to the ghost.

Ask to snuggle on the bed, and kiss him.

Try for a baby every night until you start throwing up. You will now have a ghost baby that you can control.