From the Guardian, one possible theory for the source of Akin’s idiot ‘science’:
The idea that rape victims cannot get pregnant has long roots. The legal position that pregnancy disproved a claim of rape appears to have been instituted in the UK sometime in the 13th century. One of the earliest British legal texts, Fleta, has a clause in the first book of the second volume stating that:
“If, however, the woman should have conceived at the time alleged in the appeal, it abates, for without a woman’s consent she could not conceive.”
This was a long-lived legal argument. Samuel Farr’s Elements of Medical Jurisprudence contained the same idea as late as 1814:
“For without an excitation of lust, or the enjoyment of pleasure in the venereal act, no conception can probably take place. So that if an absolute rape were to be perpetrated, it is not likely she would become pregnant.”
This “absolute rape” is not quite the same as Akin’s “legitimate rape”. Akin seems to be suggesting that the body suppresses conception or causes a miscarriage, while the earlier idea of Farr relates specifically to the importance of orgasm. Through the medieval and early modern period it was widely thought, by lay people as well as doctors, that women could only conceive if they had an orgasm.