Until such a time comes where people are forced to be hooked up to others as some bizarre form of life support, until such a time where it becomes moral to force others to donate blood and organs to save a dying person's life, until then the morally consistent option is pro-choice.
At it stands pregnancy is life-threatening. It may be less so with modern medicine, even so, abortion is a safer alternative to pregnancy. We, all of us, have the option of opting out when something is life-threatening. No one will force you into a car, no one will force you onto a roller-coaster, nobody will make you smoke, nobody will legally impose upon you a threat to your life without your consent. Nobody can even make you undergo a life-saving operation against your will (if you're a coherent, mentally stable adult, anyway).
So why is it that pregnant women are considered by some to have less bodily autonomy than somebody who refuses to donate blood? If someone giving blood every day, being nauseous, experiencing terrible pain, handicapping their ability to do X for some months at a time, and undergoing permanent physical changes all while risking their life to do so will save a STRANGER dying in a hospital... why should it be legal to give them a choice? Clearly it's better if we force people to risk their lives for strangers, force people to give up their time and health for strangers.
What is the difference between forcing a woman to have her body used as an incubator, against her consent and will and despite the risk to her health, why is that okay, when it will most likely never be permissible in our society to force someone to donate blood, over and over again no matter how this impacts their daily life, just so that someone else might live?
To me that is the only defense necessary. Simply, we do not force others to risk their lives without their consent, to refuse to give women the option to opt-out of a life-threatening experience is cruel and inconsistent with both the current state of our laws and the common morals that we see throughout society.
If we should not force everyone to risk their lives, to sacrifice their health, time, life, and plans for others, then we should not force women exclusively to do these things. Just because someone is dependent on you for life does not mean that your ability to lead your own life, to rule your own health, should so blatantly be disregarded in favor of theirs.
Why does the woman's life suddenly become irrelevant, just because the stranger that depends on her for life isn't born yet?