Two scientifically established facts will constitute the premise for this discussion:
First, an embryo is never a part of the body of the mother.This fact is proven by in vitro fertilization combined with surrogate motherhood. In that situation, the biological mother never has the baby inside herself and the surrogate mother receives and carries a child with whom she has no genetic relationship, stipulated by contract to be someone else’s. In fact, the womb donor need not be of the same species with the embryo. Thus, to save an endangered species named gaur in India, gaur embryos were gestated by domestic cows.
Second, from conception to death the human organism passes through stages of variable autonomy. It is in a state of full dependency before birth, which is then gradually reduced until full autonomy is achieved. The quality of human being does not change with the autonomy level. The autonomy is reduced again in old age and can be lost completely after an accident. If the loss is reversible, the organism is in the same condition as before birth. There is no difference in cognitive ability between a fetus and a patient placed in induced coma to save his life (the fetus has, however, a better sensory capacity). For a short time, between conception and implantation the organism is essentially autonomous. Interestingly, the cells formed during that stage, following the first division, are differentiated and oriented, a discovery that prompted the investigating scientist to state: “Your world was shaped in the first 24 hours after conception” and “(t)he mammalian body plan starts being laid down from the moment of conception.” 
The scientific data thus show that abortion, at any stage, terminates a human life. Perhaps this is why proponents of abortions sometimes insist on discussing marginal cases. Such a case is the termination of a pregnancy resulting from rape or incest. The argument used to be invoked 25-30 years ago by Mario Cuomo, then Governor of New York, to support his pro-abortion stand. Of course, the argument could be neutralized by the question: –What percentage of abortions are because of rape? The answer, known by everybody, is that the percentage is very small. Then, the next question: –Will you support a ban on abortion with pregnancies resulting from rape as the only exception? would expose the argument as a red herring.
The question of rape and abortion was asked of a pro life candidate for the US Senate in Missouri, Todd Akin, in a TV interview in August 2012. He answered it most ineptly, seeming to suggest that there are too few pregnancies from rapes to warrant all the fuss. He also used the confusing expression “legitimate rape,” much criticized after that. He may have meant to say “legitimate claims of rape.” (After all, Norma McCorvey, the claimant in Roe v Wade, admitted after conscience made her a pro life activist that her claim of rape had been a lie.) The incident would not warrant further attention, except that it has brought the subject of abortion after rape to the attention of the press and the public.
There is one study of rape and pregnancy quoted by a newsman that commmented on Akins’s miscues. It states that the pregnancy rate from rape is 5.0% among victims of reproductive age (aged 12 to 45) and that among adult women an estimated 32,000 pregnancies result from rape each year. It follows that there are 640,000 recorded rapes of adult women in the US annually. (Official statistics gave, however, 260,000 reported rapes and attempted rapes in 1995.) In a small sample for which the decisions (aborting, keeping the baby, giving it for adoption) of pregnant rape victims were analyzed, however, the majority of subjects were adolescent. In the sample, moreover, half of the subjects chose to abort the child. Eight years after the study cited, it was reported that the incidence of rape had decreased significantly in urban areas (to 0.4 per 1000 people from 2.8), but not in other jurisdictions. It was noted, however, that rape is one of the most underreported crimes.
We are facing a difficult situation. On one hand, although a small fraction (possibly 2% of the abortions performed), the unborn babies resulting from rape are still human beings and abortion takes the life of half of them. On the other hand, the trauma of rape is hard to assess by anyone else but the victim. In old days it used to be called “a fate worse than death.”
The societal attitude toward rape has changed, however, and the crime has been treated more and more leniently.. Thus, in 1977 Justices White, Stewart, Blackmun, Stevens, Brennan, Marshall and (in part) Stewart, over the dissent of Justices Burger and Rehnquist, forbade execution for the rape of an adult woman under aggravating circumstances. (The culprit had fled jail where he was held for murder, rape, kidnapping and more. Before he was recaptured, he raped a woman while committing armed robbery “and other offenses.”) Notably, the rape victim was 16 years old, but was deemed an adult because she was married. Wrote Justice White: “That death is a disproportionate penalty for rape is strongly indicated by the objective evidence of present public judgment, as represented by the attitude of state legislatures and sentencing juries.”
More recently, the Supreme Court (Kennedy, Stevens, Bader Ginsburg, Souter and Breyer in the affirmative, Alito, Roberts, Scalia and Thomas dissenting) struck down the imposition of the death penalty for the rape of a child. Justice Kennedy wrote that there was “a distinction between intentional first degree murder on the one hand and non-homicide crimes against individual persons,” even such “devastating” crimes as the rape of a child on the other. (The defendant had raped an eight-year old girl so savagely that she needed emergency surgery.)
It was reported that in 1996 the average jail time for rape was 65 months.
Strictly on principle, abortion and rape could not be considered of equal gravity, because recovery is possible for the victims of the latter act but not of the former. In social and political terms, however, legislation banning abortion might be unreachable without the rape exemption. In that case, the opponents of abortion have to choose between purism and pragmatism (absolutist vs flexible approach).
The exemptions for rape and even more so for rape and incest have an added drawback, however, because the real criminal is not punished appropriately. A baby is killed and the rapist is jailed for a few years, or the “wicked uncle” undergoes psychological counseling. A legislative compromise on the matter must require that in case of rape-caused abortion, the rapist be tried both for the rape of the woman and the murder of the baby. Besides reemphasizing that a human is killed in abortion, this adaptation of the law would assure a more appropriate punishment for the culprit. Even though these days they also have been reduced, the jail terms served for murder are about 50% longer than those served for rape (and the two sentences can also run consecutively). This approach will also have a deterrent effect on the crime of rape.
References and Notes
. D. Fărcaşiu, Attitudes, Positions, and Actions Taken in Response to the Use of Materials from Nonconsenting Humans, Particularly the Unborn, in Medical Research and Practice, 2006 (posted here on 08/11/2012, under Religion)
. H. Pearson, Nature, 2002, 418, 14.
. M.M.Holmes. H.S. Resnick, D.G.Kilpatrick, C.L. Best, Rape-related pregnancies: estimates and descriptive characteristics from a national sample of women, Am. J. Obstet. Gynecol, 1996, 175, (2) 320 (They report 32,101, but an estimated value cannot be that precise.)
. L.A. Greenfeld, Sex Offenses and Offenders: An Analysis of Data on Rape and Sexual Assault; http://www.mincava.umn.edu/documents/sexoff/sexoff.html
. D.A. Farenthold, Statistics Show Drop in U.S. Rape Cases, The Washington Post, June 19, 2006.
. Supreme Court of the United Sates, Coker vs Georgia (decided June 29, 1977); http://www.law.cornell.edu/supct/html/historics/USSC_CR_0433_0584_ZS.html
. Linda Greenhouse, Supreme Court Rejects Death Penalty for Child Rape, The NY times, June 26, 2008; http://www.nytimes.com/2008/06/26/washington/26scotuscnd.html?pagewanted=all
. S. Rafael, M.A.Stoll, Why Are So Many Americans in Prison? http://www.law.berkeley.edu/files/Steven(2).pdf