Ghost baby

In The Sims 3, it is possible to "Try for Baby" with a ghost and spawn a ghost baby.

Making a Ghost Baby
To achieve a ghost baby is the same as a regular baby. 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.

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.

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. There is no known signs during the pregnancy that the Sim has conceived a ghost baby or regular Sim baby. There is a 50% chance that your Sim will conceive 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. Or thought you can sometimes both can ghost babies.

What Ghost Babies are
Ghost babies conceived either from a ghost female or by a ghost male are NOT immortal. They will age just like every other Sim, and even die of old age. They cannot, however, die of starvation, drowning, fire, meteors, mummy's curse, or electrocution. When the ghost "dies" of old age, the game will actually say that the Sim has moved on to the Netherworld and the player no longer has control of the ghost.

Ghost babies are like ghosts in every other way, they even have their own tombstone (which appears in their invetory) and appear as dead on the family tree.

What Ghost Babies Look Like
If the baby looks like a normal Sim, it will be completely normal in every way. The baby can, however, inherit the skin, hair, eye color, and other traits from the ghost parent. If they have the appearance of their ghost parent/parents, then they will not appear in the family's picture.

The ghost baby will have the same ghost color of the ghost parent and will look exactly like a normal ghost. If the ghost baby has two ghost parents, the baby will be the ghost type of one of the parents. There is no way to mix colors of two ghosts.

The player can still edit these ghost Sims in Create a Sim with the dresser and mirror, but in there the ghost will appear as a normal Sim.