Ta-Nehisi and Megan tackled this topic a while back. Here's TNC:

Slave-masters often allowed--indeed encouraged--slaves to engage in acts common among people. Slaves married. Slaves were baptized. Slaves were converted to attend Christianity--and even attended white churches, at times. Slaves and masters exchanged gifts on Christmas. Slaves were allowed to hire themselves out and buy their own freedom. Slaves were manumitted by masters. The point is that what you see in all of that is something more complicated than "Are Africans people?" The better question seems to be "Are black people equal to whites?"

But more than that, core reason an abortion/slavery comparison falls down lay in the actions of the enslaved, versus the inability of action amongst embryos.

Abortion is a debate between two groups over the ultimate fate of embryos.  The Anti-Slavery fight was a violent struggle between two groups over the fate of the enslaved, but with the enslaved as indispensable actors. Unlike embryos, black people were very capable of expressing their thoughts about their own personhood, and never held it in much doubt. Whereas the fight against abortion begins with pro-lifers asserting the rights of embryos, the fight against slavery doesn't begin with the abolitionists, but with the Africans themselves who resisted.

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