Joker wakes up from his dream and finds that he’s pregnant. I’ll repeat: the Joker wakes up in the morning and discovers that he’s PREGNANT. I didn’t make that up. Just look at the picture below.
So the Joker goes to the hospital and PREPARES TO GIVE BIRTH. Then he winds up throwing up a mud monster instead. A violent, gun-filled chase commences, but then it is discovered that the mud creature has morphed into a child-sized version of the Joker.
The Joker accepts the thing as his son and shows him off to Zatanna, taunting her over the family she could have had with him.