Akin didn't say that. First of all, the word he meant to say is forcible instead of legitimate. Paul Ryan and Akin co-sponsored an abortion-related bill that would have limited federal funding for abortions to victims of forcible rape," rather than just rape. The FBI's definition of rape is known as "forcible rape," and it is used to distinguish between statutory rape and other kinds of rape.
Akins never said that rape could not happen during forcible rape. This is what he said during an interview on the Huckabee show:
Huckabee asked Akin whether he was talking about "forcible rape" when he used the term "legitimate rape."
"I was talking about forcible rape, and it was absolutely the wrong word," Akin said.
Akin said he understands that women can become pregnant from rape. "I didn't mean to imply that wasn't the case," he said. "That does happen."
“This is what he said during an interview on the Huckabee show”
This was after the firestorm broke. Go watch the original interview.