The entire bodily autonomy argument is predicated on the understanding that the adult woman is, regardless of circumstances or law, always autonomous in will, and always retains choices--even if some of those choices are unethical or desperate. She may not be able to simply will her body into rejecting the child within her--she doesn't actually have that kind of control or governance over her body, any more than she was able to prevent conception by willing herself not to ovulate. But she has the ability to take actions directed at her own body, and she has the power to take actions directed at the child within her, who is utterly vulnerable to her.
When a woman is pregnant, neither she nor her child are autonomous of one another. The cost of freeing the woman from the bodily costs of pregnancy is the utter violation of the body of the unborn child.
It is a calculus that ultimately comes down to the strong taking precedence over the weak, and it undermines the very basis of the ethical calculus that women depend upon to safeguard our own lives and bodies from violation and violence.
Her child is in a dependent position. Traditionally, western ethics has favoured protecting those we recognise as dependent on the goodwill of the strong of society. This ethical insight has driven the development of child protection services, animal welfare societies, elder's rights groups, disability rights activism, and, for that matter, feminism.
Pro-choice activism undermines the foundational philosophical origins of feminism, which is predicated upon the ethical tenet that weakness of body or lack of political/social power does not invalidate a claim to equal human rights.
Foremost of which is a right to protection from acts of intentional violence.
Making room for the human rights of others always requires sacrifice. It always requires that we (the privileged) accept risks we would otherwise be insulated from and accept that we are interdependent.
The good of others requires more from me than simply not acting in a directly hostile manner myself. It might, at times, require that I intervene to defend the weak or victimized even if that means a business collapses, a family that is profiting from injustice is harmed, people die because a medical advance takes more time (just think how much could be accomplished if we didn't have so many restrictions on human studies!), and so on.
There are few things as intersectional as the meeting place between the rights of children, including unborn children, and the rights of women. The relative privilege lottery in that intersection falls solidly on the side of adult women. We may have other battles and other disadvantages, but we are still not as vulnerable as the developing child, who has none of our visibility, voice, or freedom of action.
Image via https://pixabay.com/en/oppression-women-violence-barbie-458621/