I think when they were in jail they had s*x on the rooftop or something and that was the day before his release. So he got prego when they did the deed that day. Since it was the day before his release he woke up early and left prison.
If we're trying to make this story realistic, I think he has a sex-linked chromosomal disorder where he has both female and male sexual organs. That wasn't explained in detail though.
I think i missed something- how did he get pregnant huh