Game guide:Having a ghost baby

There are many ways for obtaining a ghost baby. However, a ghost baby requires some methods that a regular baby does not. A Female Sim must interact with a ghost every night when it appears near its urn or tombstone and build a relationship to at least "romantic interest", then when at home, try for a baby.

Ways of achieving a ghost baby
The trick is to get the ghost to come home with the Sim, first. There are several ways to do this:
 * 1) Steal their grave (if the Sim has the Kleptomaniac trait) and put it on their current home lot.
 * 2) Marry the ghost to add it to the household.
 * 3) Kill off a Sim and make a playable ghost by accepting the "Oh My Ghost!" opportunity after the Sim has died.
 * 4) Go to the graveyard at night and invite the ghost Sim to the playable Sim's house. (The ghost will disappear by morning.)
 * 5) Build the relationship between the ghost and playable Sim until you have the option to pick up the grave and move it to your Sim's home lot.
 * 6) Have a Sim throw a Ghost Potion during pregnancy.

Having the ghost as part of the household makes this process much easier because they are available to interact with them at all times of the day. If you don't have it in your active household (meaning you can't click on the ghost to see it's needs and whatnot) then you can only interact with it between about 11:30 PM to 5:00 AM Sim time.

Once the Sim is pregnant, the pregnancy will proceed as normal. Whether or not the baby will be a ghost is decided at birth, which means you may save before the baby is delivered, and reload the game untill you receve a ghost. There is a 50% chance that your Sim will produce a ghost baby (that is, unless, both parents are ghosts, then it will always be a ghost baby). Currently there is no known way of increasing the chances of having a ghost baby instead of a regular Sim baby, though getting the fertility treatment Lifetime reward may help because one twin can be a ghost while the other is a regular Sim.