Only about 1% of abortions cite rape as a cause.* Probably less. (I see some cites which take it down to 0.5%, though these are pro-life sites and might not be trustworthy.)
There are 6,053,840 pregnancies per year in the US, of which 1,200,000 end in termination (abortion).**
The incident of pregnancy from a single, unplanned sexual encounter seems to unclear,*** but I’m going to go with 5% because the majority of rape victims seem to be in their prime for fertility and that seems to be the top number.****
About 0.4 in 1000 people (including men, I presume) are raped each year.***** Presuming that nearly all of those are women and that women are about half the population, then that is about 62,000 women per year.
Of women who become pregnant due to rape, about 75% choose to have the child anyways.****** (Note that this is a pro-life site, so not necessarily trustworthy.)
From these statistics, we would expect that there are (62,000 * 5% =) 3,100 pregnancies caused by rape each year if rape does not increase nor decrease the chance of pregnancy from a single act of sex.
From the incidence of termination, however, we would expect that there are 12,000 abortions from rape victims (or 6,000 if you take the 0.5% number), and a total of 48,000 pregnancies since most women do keep the baby (or 24,000 at the 0.5% number). Since that would be nearly the full population of rape victims, I think we can safely say that the 0.5% number is more likely. Even still, 24,000 pregnancies among 62,000 women is far higher than the expected 3,100.
The statistics on NationMaster give the incidence of rape in the US as close to 90,000 victims per year.******* This still only works out to 4,500 expected pregnancies.
Rape is often said to be under-reported, and that is almost certainly one factor in the disparity here. Of course, the 0.5% number is also reliant on self-reporting of rape, so you would expect the same result there, and yet I don’t see any cites going lower than 0.5%. While I’d have been suspicious of the study which has been brought up that suggests that rape is twice as likely to end in pregnancy as consensual sex, I might have to say that the reality may well be pointing in that direction. I have no idea why. All I can suggest is that Mother Nature is an evil master.