Justine Larbalestier provides some context for Republican MO senate nominee Todd Akin statement that, "from what I understand from doctors [pregnancy from rape] is really rare. If it's a legitimate rape, the female body has ways to try to shut that whole thing down."
It turns out that this belief in magic sperm-rejecting vaginas was the kind of thing that was believed in 1785, when Samuel Farr argued in his groundbreaking treatise on law and medicine that:
Samuel Farr, in the first legal-medicine text to be written in English (1785), argued that "without an excitation of lust, or enjoyment in the venereal act, no conception can probably take place." Whatever a woman might claim to have felt or whatever resistance she might have put up, conception in itself betrayed desire or at least a sufficient measure of acquiescence for her to enjoy the venereal act. This is a very old argument. Soranus had said in second-century Rome that "if some women who were forced to have intercourse conceived . . . the emotion of sexual appetite existed in them too, but was obscured by mental resolve," and no one before the second half of the eighteenth century or early nineteenth century question the physiological basis of this judgement. The 1756 edition of Burn's Justice of the Peace, the standard guide for English magistrates, cites authorities back to the Institutes of Justinian to the effect that "a woman can not conceive unless she doth consent." It does, however, go on to point out that as matter of law, if not of biology, this doctrine is dubious. Another writer argued that pregnancy ought to be taken as proof of acquiescence since the fear, terror, and aversion that accompany a true rape would prevent an orgasm from occurring and thus make conception unlikely.
(Quote from Thomas Laqueur's Making Sex).
Justine notes that Farr's work was written in the same century in which Mary Toft was widely believed to have given birth to rabbits.
So yeah, that the kind of "science" that Todd Akin will bring to the Senate.