I haven't done a ton of research myself but it does seem logical that less pregnancy would result from rape less frequently than other intercourse. A woman's body need the right chemistry to support pregnancy. A stressed out woman is less likely to get pregnant. I don't know what else could cause stress more than rape.
Here is an article that supports this suggestion written by Dr. John C. Wilke, M.D.
http://www.christianliferesources.com/article/rape-pregnancies-are-rare-461
"Finally, factor in what is certainly one of the most important reasons why a rape victim rarely gets pregnant, and that's physical trauma. Every woman is aware that stress and emotional factors can alter her menstrual cycle. To get and stay pregnant a woman's body must produce a very sophisticated mix of hormones. Hormone production is controlled by a part of the brain that is easily influenced by emotions. There's no greater emotional trauma that can be experienced by a woman than an assault rape. This can radically upset her possibility of ovulation, fertilization, implantation and even nurturing of a pregnancy. So what further percentage reduction in pregnancy will this cause? No one knows, but this factor certainly cuts this last figure by at least 50 percent and probably more. If we use the 50 percent figure, we have a final figure of 225 (or 370) women pregnant each year. These numbers closely match the 200 that have been documented in clinical studies."
Please read the whole article for more supporting data