>You would have the legal right to back out of donating a kidney, even if you already agreed to do it and it would save someone's life. Similarly, the argument goes, you should have the legal right to have an abortion (or at least induce labor, if viability is possible) at any point during a pregnancy.

that's not really a good analogy because prior to donating the kidney, the kidney recipient isn't dependent on your kidneys, whereas at the moment of conception the zygote/fetus is dependent on the mother's body. A better analogy would be being able to take back the kidney after it's been donated, although that's also flawed because you're not really giving anything up when you're getting aborted.