What happened to Noah at his birth?

Pa Joad delelivered his first child because there was no help to be found for the birth. Noah was somehow deformed, and the midwife had to mold his little body back in shape. Pa always felt guilty about this and has a special love for Noah that he doesn't have for the others.

Noah "left the impression of being misshapen, his head or his body or his legs or his mind; but no misshapen member could be recalled" (Chapter Nine)