http://www.jessebering.com/...
Basically, the scientists studying this have concluded that there is a small increase in the incidence of pre-eclampsia, a medical condition during pregnancy that could result in a spontaneous abortion, that is due to the pregnant woman not being sufficiently accustomed to the sperm of the father. The idea, from an evolutionary standpoint, is that the probability of the father of a child sticking around to raise the child correlates to the amount of sperm that he had deposited into the mother beforehand over a long period of time - which is a reasonable correlation as a wham-bam-thank-you-m'am, or yes, rapist, father would certainly be more inclined to not stick around - and that in pre-historic times, having an absent father was correlated with a high level of death to the child & mother, as well as that a pregnancy for the mother is a very large investment of her fertile time, even if she were to survive - with the evolutionary result that a woman who could have a higher probability of spontaneously aborting the product of copulation with such a man would have a slightly higher evolutionary fitness, and hence this trait would be seen in the general population of women. I suppose some kudos should be given to a rapid anti-evolutionist for using evolution to prove his point, even if he grossly applied this theory!
Of course, this mechanism has nothing to do with the psychological attitude of the woman with respect to her impregnator - for a woman who does not have relations with her man until officially trying to get pregnant (which would cover the situation in which her man used a condom), or who gets artificially inseminated with some other man's sperm, or who has a 1 night stand, etc., the chances of her having a successful pregnancy are the same as if she were impregnated by a rapist.
Also, this mechanism is only a slight factor in the total probability, so for example, if the chances of a woman at the right time in her cycle of being impregnated by her regular man (and with his semen having been deposited in her for a long time) are say 50%, then the chances of rapist doing the same might be something like 40% - hardly a mechanism that can be depended upon to ensure that a woman does not bear the result of a rape. And in any case, if the rape were incest (which is a form of statutory rape - yes I know not "forcible"), the victim could very well be accustomed to the perpetrator's sperm.
Finally, even if a woman could magically tell her uterus to spontaneously abort, there would be the medical problem of her aborting while away from a medical facility that could minimize complications - something that a pre-emptive abortion could ensure. I suppose that Akin and whoever clued him in on this somehow thought that the spontaneous abortion would be one of a tiny embryo very early in the pregnancy that would not have any health effects for the mother.
So you see, even though it may seem that these whackos just make shit up, sometimes they are getting the kernel from some correct source - it's just that they twist it around to make it fit their worldview.