Easy, define a form of bodily autonomy that permits forcing conscious action upon an individual (this is the basis for many laws^1^ ), but not prohibiting the individual from engaging in an action to override an already occurring unconscious process.
This is necessary because the former is the description of what many morally accepted laws already do, and the latter is a description of what prohibiting abortion is.
In other words this is the exact definition that we need to show is correct to justify abortion on the grounds of bodily control.
Except we can’t, and it’s obvious why. Saying “you must do X” is clearly stronger than saying “you cannot stop Y from continuing to happen”. So we already accept a greater violation of bodily autonomy as good, and the abortion defence is actually contradictory.
We can resolve this by rejecting one of the premises. So which one do you want to reject? The one that is the basis for societal rules, or the one that allows killing humans?
As I already pointed out the bodily autonomy argument is essentially completely rejected in ethics, it’s only popular because of Thompson’s deeply flawed and overly simplistic paper (primarily because it already assumes that such a form of bodily autonomy already exists).
Consider the fact that if you are in a circumstance were someone else depended on minimal effort from you for survival, saying you did not want to provide it is not a legal defence. You can’t just let your child drown in a 2ft pool, and claim that your right to bodily autonomy allows you to withold conscious support. You intuitively know that it is immoral simply to withhold life-saving actions, and so does everyone else in society. The only reason why fetuses have an exception is that they don’t appear human, despite satisfying all the necessary conditions. It is simply psychologically easier, much like how it’s psychologically easier to kill strangers who look differently to you than your friends or relatives.