Sorry, if Akin can refer to a 1970's report by Willke, then this is easily as valid. WIllke, remember, wrote that teh odds agaist against imprenation in rape were "millions and millions one." Regardless, if the topic was investigated then the results of that investigation would be available.
This one ( http://www.ncbi.nlm.nih.gov/pubmed/8765248) concludes that just over 30,000 pregnancies result from rape each year. (Seems really high to me) Are 30,000 pregnancies rare? Several other analyses suggest that rape produces more pregnancies than consensual sinlge acts fo intercourse. The details are outlined at http://blogs.scientificamerican.com/context-and-variation/2012/08/20/here-is-some-legitimate-science-on-pregnancy-and-rape