I'm pro-choice, because:
1) A woman's body is her own. Nobody, no government, no court, can force a woman to gestate a pregnancy to viability if she does not also agree to do so. It is unfortunate that human reproduction happens inside women, but because that is the truth, anything less than a pro-choice stance means invading her personal privacy, bodily autonomy, and forcing her to risk her emotional and psychological well-being, her physical health, both during pregnancy and post-birth (in some cases, even her lifelong health), and possibly even her life.
2) Neither pregnancy nor a child should be a punishment. Sex is a good thing, and nobody should be punished for having it. Mistakes happen- let he who is without a mistake step forward and cast the first stone!- and we should not criminalize them. I firmly believe that forced pregnancy of unwanted children can lead to parental resentment (which may take the form of abuse and poorer care) and floods our adoption and foster care systems even fuller than they already are. As the pro-choice slogan goes: "Every Mother A Willing Mother, Every Child A Wanted Child", and I truly believe that reaching this goal is good for women, good for men, good for children, good for society, and the compassionate choice.
3) Criminalizing abortion (which is typically what the pro-life movement advocates) does not reduce rates of abortion. Study after study confirms that the abortion rate stays fairly static whether or not abortion is legal, and in fact, the countries with the lowest rates of abortion are those where it is legal.
4) Criminalizing abortion raises infant and maternal mortality rates. Without access to safe, legal abortion, women die, either in attempts to self-induce abortion, or at the hands of unscrupulous or untrained abortionists. Similarly, attempts to curb access to abortion have been shown to have negative consequences; for example, mandatory waiting periods lead to an increase in more second-term abortions (which are more risky and expensive) rather than to decrease abortion.
5) The logical conclusions of criminalizing abortion is absurd. If a fertilized egg that has yet to implant is considered to legally be a person, then every miscarriage will need to be investigated to ensure no foul play. How will we know if a woman who fell down the stairs truly had an accident, or if she was attempting to induce a miscarriage? Women who commit suicide when pregnant may face criminal charges instead of the mental healthcare they require. Who determines whether or not a high-risk pregnancy is worth aborting, if there's a 50/50 chance both mother and child will survive- even if the mother doesn't want to risk her life? One need only look to Romania under the rule of Nicolae Ceauşescu, where a total abortion ban (and ban on contraception) lead to "a kind of pronatalist police state, where women were subject to random gynecological exams and all miscarriages were investigated" (Michelle Goldman in "The Means of Reproduction", pg. 80).
6) Societies thrive when women control their reproduction (particularly when combined with gender equality- something that *can* be present in pro-life circles, but overall is much more closely aligned with the pro-choice side). Not only does being able to have the number of children you want, when you want and are ready for them, reduce poverty (as Hitchens so beautifully says: "The cure for poverty has a name. It's called the empowerment of women. If you give women some control over the rate at which they reproduce, take them off the animal cycle of reproduction to which nature and some religious doctrine condemns them, and then if you throw in a handful of seeds, the floor of everything, in that village, not just poverty, but health and education, will increase."), but we have also observed that countries that have higher infant and maternal mortality rates are those which are more politically unstable as well. Some have even gone so far as to link abortion access to crime reduction. Happy, healthy, wealthy, peaceful societies are those where the citizens are well-educated about a vast array of reproductive choices and have access to an arsenal of tools- abortion included- to help them ensure whatever choice they make.
7) Pro-life policies create more unplanned pregnancy. While there are undoubtedly some great pro-lifers who embrace comprehensive sex education, widespread use of contraception, universal healthcare, and welfare and aid policies, they're definitely in the minority. All the major pro-lfie organizations eschew policies that have been proven to reduced unplanned pregnancy (and hence the need for abortion). Pro-choice-affiliated policies seek to reduce the need for abortion in the first place, and have been proven to be effective in doing so.