I have to say that to compare a fetus to a parasite doesn't work well as a metaphor, again from a scientific perspective. I don't know if the following will be appreciated, but I work in a molecular biology research lab and am in school to become a biologist, so I have some experience on this point. If anyone else has experience here, correct me if I oversimplify or overstate anything too much.
A parasite lives in a symbiotic relationship with the host such that the host is disadvantaged and the parasite is advantaged.
However, from an evolutionary perspective the mother is by no means the host to the fetus because the fetus does confer an immense advantage to the mother; a definition of evolutionary success is to survive long enough to reproduce young, and thus to pass on one's genes. So, again from a scientific perspective, treating humans as any other animal, offspring are essential because any organism which fails to reproduce has effectively cut itself out of the next generation. Whatever the traits that led that organism to be unable or unwilling to reproduce, they were an evolutionary disadvantage because they self-select themselves out of the gene pool. Does that make sense?
Now, what if the abortion under consideration is calculated, in the sense that to have offspring at the present time is not advantageous (let us say her ability to ensure its survival to adulthood is at this time compromised), but that at some point in the future if the mother is able to continue directing her resources towards herself she will be able to sustain viable offspring, perhaps several as opposed to only one? Or, alternatively, it is a question of an impoverished offspring now versus a healthier or better-situated offspring later? In this case, it might be appropriate to say that the mother who is able to make the better judgement call may succeed genetically in the long run.
The mother who does not produce offspring always loses genetically; however, this does not necessarily rule out abortion as a reproductive strategy in some circumstances.
I'm guessing that some people may not like a way of looking at this which views the goal of life as reproduction, life as non-sacred, and the relationship between parents and offspring as being a matter of self-interest and genetic survival. Remember that this is a purely practical and scientific perspective on who survives and who doesn't; people are perfectly free to tack on values and meanings, or even to reject strongly-supported evidence because they don't want to think about it. For example, I myself compartmentalize between what the evidence suggests and implies, on the one hand, and then what I believe because of faith and in the teeth of the evidence, on the other hand.