but unless the mother was not informed (minor/mental illness) or did not consent,

This is already a significant retreat from your previously stated position. ("not unless it's going to kill you" after 24 weeks)

The complication here is that a responsible, consenting adult tacitly accepts giving up her bodily autonomy (or accepts a risk of doing so) when she has sex.

That's a hell of an assertion. I don't really see any reason to accept it as other than a normative statement of what you wish would happen.

That's precisely the same reason men are required to pay child support even if they didn't wish for a pregnancy. (Yes, I see the asymmetry; yes, it sucks).

As you say, there is an asymmetry. Garnishing a wage is a bit different, and seems appropriate to me.

Case-by-case reasoning is probably a good thing in these circs,

Yes, it is, so long as it is reasoning rather than assertions that this case is different. We have to specify how it is different, and how those differences make a difference. The easiest way for me to do this is to use analogies. This is dangerous of course, as one must keep in mind that they can ignore relevant differences while emphasizing surface similarities.

So, in this case the relevant specialness you're calling out is that a risky activity was knowingly engaged in that created a person who needs life support for some time, as well as care and feeding far after that. So I'm going to try to set up an analogous situation, but without sex being the act (which I think is irrelevant) coming into the mix. This will also mean another difference: the person will not be "created" except metaphorically from a preëxisting person. I personally don't see how that would be relevant, but I suppose it is possible for others to disagree.

Suppose a person is driving, and crashes into a pedestrian. This ruptures the liver of the pedestrian. A partial transplant of the driver's liver will save the pedestrian's life. Is the driver expected to donate their liver? Should it be required by law?

Note that the donor's death rate for this operation is under 1%. When we compare this to the statistics for maternal death, we see it is similar to WHO's 2005 estimate of world average of 900 per 100,000, though developed regions have it far lower at 9 per 100000.