Just wanted to mention that the shooting script points out that Jason was born PREMATURE! I think that explains it all, too bad they didn't put it in the movie so everyone could be enlightened right away.
"A hero is an ordinary individual who finds the strength to persevere and endure in spite of overwhelming obstacles." Chris Reeve
"Whatever comes our way, whatever battle we have raging inside us, we always have a choice. It's the choices that make us who we are, and we can always choose to do what's right." Peter Parker