Ghost baby

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

The Sims 3
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. The ghost child can use their tombstone to move on to the Netherworld by choice. If the player chooses to resurrect them again, their age will be reset to the maximum number of days before they age up, which can be very useful. Ghost babies are like ghosts in every other way, such as appearing as dead on the family tree.

Ghost babies can also be conceived in Generations by having a man and woman drink the ghost potion and try for baby while they are still ghosts.

The Sims 4
Ghosts in The Sims 4, regardless of age or gender are unable to conceive a baby. There is no "Try for baby" interaction for ghost-ghost or ghost-sim couples. The only ways to get a ghost child is to create one in CAS through ghost parents or getting the "death" outcome when wishing for a child at the wishing well.

Appearance
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 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 with usually glowing white eyes. However, if Supernatural is installed, the ghost Sim will keep their ghostly appearance.