The principle argument I've seen for Pro-Choice is Body Autonomy. Each person has the right to their own body and to determine how it is used. You can't force someone to donate blood or organs, even to save another life, and as such, you can't force them to carry a pregnancy to term. The principle argument I've seen for Pro-Life is Right to Life. Each person has the right to live, and abortion denies that right. It is also morally wrong to take a life. Historically, women have been denied Body Autonomy as a means to reduce their power and disenfranchise them. Whether through ownership by parents or their spouse, as a chattel, or through being denied contraception and abortion, women in most countries around the world have less Body Autonomy than men, both de jure and de facto. Pregnancy is also dangerous, and even the best pregnancy presents some level of threat to a woman's life. This discussion usually focuses on abortion, but really the two sides are arguing about different things. Pro-Life advocates are arguing that abortion is morally wrong because it is killing a living person. Where personhood starts is another aspect of this, but I think nearly all Pro-Choice advocates would agree that beyond some distinguishing point the fetus is alive and killing it is morally wrong. Pro-Choice advocates then take the next step to say that, while it is wrong, it is still necessary. And that's where most debates stop, but they really shouldn't. The next question should always be "why is it necessary?" I think it comes down to three things: lack of foreknowledge, lack of effective contraception, and emergencies. 1) We lack foreknowledge because we don't have standard, safe process in place to give parents all of the information they need to determine if they should both have and keep a pregnancy. That includes genetic testing, adequate health education, relationship counseling, etc. We need to focus our efforts on building a comprehensive system to ensure that all pregnancies are desirable pregnancies. The state of women's healthcare in general in the U.S. is atrocious, but the state of men's reproductive healthcare isn't much better. If we want to solve the problem of unwanted pregnancies, we need people to be informed and willing participants in parenthood. 2) We lack effective contraception because, even when it's available, many people choose not to use it due to the side effects. Most are uncomfortable, many cause hormonal issues, and some can literally causes bodily harm. We need safe, universal contraceptives - that means for everyone, both men and women. And that pretty much rules out all hormonal and implant contraceptives. If we want to stop abortions, we need people to be informed and willing participants in contraception. 3) Emergencies are going to happen. Whether the mother's life is threatened or the pregnancy is due to rape or incest, there will always be a need for surgically removing a child before birth. If we want these events to stop being abortions, then we need to fund research into new methodologies for performing surgically assisted live births. When there is no longer ever a need to kill the baby in the process of terminating a pregnancy, then we can stop having abortions. All three of these things require that we fund and support medical research into human reproduction. The main problem is that many Pro-Life advocates are also against contraception and sex education. And the policies and social norms that arise from those stances are the primary factors in creating new abortions. So they target abortion as something that is obviously wrong.