Book in which adult man grows inside of womb

[pnwgirl]

Hi! I can't remember the title of a science fiction book in which there are twins in utero and one is born and the other baby decides he doesn't want to be born and so grows inside of the mother until he is an adult man. They have to roll the mother around on a giant cart because she has a full grown man inside her. People start thinking he is the next messiah and they feed computers inside the womb so the man can communicate. Can't remember when it was published, maybe 15-30 years ago? Has something to do with no original sin because he was never born of woman. Thanks for the help!

 

[CassFrank]

Bumping this.
A fascinating enigma.
Hopefully it will eventually ring a bell…