I think that steven wasnt going to survive childbirth, so rose gave her gem to him. So he could very well have split into rose and his lifeless body.>>>>except rose knew she'd die, probably before she even conceived Steven.
Frankenweenie, Corpse Bride, and The Nightmare Before Christmas- a continuous story- "Every story has a beginning, a middle, and an end. Not necessarily in that order. I noticed this! This is why I love Tim burton!