In his opening statement, Mr. Carrier identified two sources of disagreement on moral issues: disputes over facts, and disputes over values. I agree with his assessment; however, some disputes do not lend themselves to easy categorization. Take, for example, the question of when a human individual comes into existence. The answer has both a factual component -- the scientifically observed principles of human development -- and a values component -- what do we mean by "human individual"?
In this rebuttal, I will focus on the ways in which Mr. Carrier and I disagree over both facts and values in the question of human individuality. I will also identify several other areas in which we disagree on the facts or the interpretation thereof.
The Human Individual
My opponent notes that there is nearly universal agreement on the value of respect for individual human existence, and from that premise he concludes that the dispute over abortion is solely a dispute about facts. I believe there is one important question of values that he overlooks, however. That question is "What do we mean by individual human existence?"
Mr. Carrier defines a human individual as a particular personality. On the other hand, I hold that the human individual is the organism that develops and exhibits the personality. Moreover, I hold that the personality cannot meaningfully be considered apart from the body.
I am not certain what kind of being the "vacant human body" (prior to the development of the cerebral cortex) could be; if s/he is not considered a human individual. Surely the body is not waiting for some outside force to inject a soul or a personality. Rather, the personality results from the interaction between the body and the rest of the world.
The body is the medium through which the mind experiences the world. As such, its development is integral to the formation of the personality. Genes, intrauterine hormone levels, maternal disease and use of certain chemicals all have the potential to shape the development of the brain long before a complex cortex is evident. Therefore, it is not true that the body is merely an empty shell before the 20th week, passively awaiting the addition of an individual personality. The body has been directing the development of the brain all along, according to the instructions encoded in its DNA -- and with modifying influences from the external environment.
The prenatal human may be currently unable to manifest a particular personality, but s/he is nonetheless in the process of developing it.
Abortions After 20 Weeks
Mr. Carrier asserts that abortions after the 20th week of gestation are extremely rare, and done only in the most extreme circumstances. He also points out that they should not even be called "abortions" because they fall outside of the standard medical definition of that term. If he wishes to use some other word, I have no objection. However, the fact remains that there are doctors who do use the term "abortion" to refer to procedures after 20 weeks' gestation in which the intent is to kill the fetus, and such procedures do occur for reasons other than to save the mother's life.
Dr. Martin Haskell of Dayton, Ohio was the first person to publicize the use of the controversial "intact dilation and extraction" or "partial-birth abortion" procedure. In an interview with the American Medical News in 1993, he admitted that about 80% of the abortions he performed between 20 and 24 weeks were "purely elective" and the rest were for "genetic reasons" such as Down syndrome or even cleft palate. 
In 1996, a reporter for the Bergen [NJ] Record found that 3,000 post-20-week abortions were done per year at a single New Jersey facility, of which only "a miniscule amount" were for medical reasons. 
Nationwide, the Alan Guttmacher Institute estimates that 1.5 percent of abortions are done after the 20th week, although they do not have figures on how many are for medical reasons.
Abortion and Population Control
My opponent argues that legal abortion is necessary to check population growth. According to his opening statement, "The ability of societies to check population growth without legalizing abortion has proven nearly non-existent: there are few countries in which abortion is outlawed or stigmatized that are not suffering harshly from overcrowding..."
However, correlation does not prove causation. The countries which experience the fastest population growth are also those in which poverty is widespread, the infant mortality rate is high, there is little to no social support system for the elderly, and contraception is either unavailable or not used. These factors also tend to correlate with high birth rates, and must be taken into account.
Slowing of population growth can be achieved without resort to abortion. In the United States, the decline in population growth was well under way by the time abortion was legalized nationwide in 1973. 
Finally, the fact that a practice helps keep population growth in check is an insufficient reason to condone it. Historically, the primary factors keeping the population in check included war, disease, and famine. While I agree that population and consumption are important issues, we can and should find non-destructive means of curbing them.
Abortion as a Benefit to Women
In closing, I wish to make two quick points about the assertion that abortion is beneficial to women. First of all, if the morbidity/mortality rates for abortion in general are in fact lower than the rates for carrying to term in general (accurate statistics on abortion are always hard to pin down), it still does not follow that abortion is safer than carrying to term for any given woman. In any event, most women do not have abortions due to medical indications. Second, I believe that Mr. Carrier was unduly dismissive when he wrote about actions that might be taken to alleviate the social and economic harm that can befall a pregnant woman. Of course, it will not be easy to build mother-friendly schools and jobs, improve adoption procedures, crack down on fathers who abandon their partners and children, and provide health care and public assistance when necessary. But once again, I argue that we have the ability, and therefore the responsibility, to seek non-violent solutions to the problems faced by pregnant women. I cannot stress this point enough: we need not accept the inevitability of abortion, just as we need not accept the inevitability of injustice toward women. To do so would be to admit that we are incapable of doing better.
Now read Richard Carrier's Second Rebuttal
3. Source: Population Estimates Program, Population Division, U.S. Census Bureau Internet Release Date: April 11, 2000