I think women should always be able to terminate a pregnancy, whatever the term. It is her body that it bearing the strain of carrying the baby, and if at whatever point she decides she no longer wishes to do that to herself, she should have the right to stop.
Whether or not that leads to the death of the foetus is not relevant to that discussion. However, it is a different and much more difficult discussion as to what should happen to a baby that is born as a result of such a termination and is viable, with or without medical intervention. In fact it's two different discussions, depending on whether or not the baby requires medical intervention.
Think of it this way. Suppose medical knowledge was such that someone's cancer could be treated using cells from a donor (with a suitable genetic match). Suppose that the treatment required the donor to have daily sessions for nine months, and take a number of drugs which affected them quite strongly. Early on in the process, the patient would have no chance of surviving if the treatment was interrupted, but later on there would be some chance, increasing right up to the end of the treatment at nine months.
Do you really think that the donor should, because they agree to undertake the treatment initially, have a legal obligation to see it through to the end regardless of what it is doing to them, physically and/or mentally?
I cannot imagine a developed legal system which would impose such an obligation on the donor.