Other posters have showed that the stress of violence does prevent many pregnancies.
Now, the problem with his statement isn't over the accuracy of it, but the use of 'legitimate' with the word 'rape'.
Are you and others saying this stuff seriously nuts? Even Akin has disowned his comments. As best I can tell, Akin himself is no longer claiming women's bodies have the magical ability to prevent rape pregnancy. Armies have used rape as a weapon and have throughout the history of warfare and occupation. A completely obvious recent example is all the German women raped by a conquering Red Army. There were thousands and thousands of unwanted pregnancies - many resulted in the suicide of the mother and obvious resulting death of the fetus.