The concept of a parasitic fetus is derived from the fact that the fetus is entirely dependent on the mother for its nutrient supply. However a parasitic relationship is a very give-take sort of relationship. Where the parasite only takes from the host, and provides nothing back in return.

For the human fetus, there exists the placenta, which is an organ which allows for the exchange of materials both from the mother to the fetus, but also from the fetus to the mother. In fact the fetus has a profound effect on the mothers biology, including inducing a variety of changes to the mothers immune response which change as fetal development goes on.

In the end, one should likely classify the relationship between a mother and fetus as a symbiotic one, albeit skewed in favor of the fetus in terms of nutrient requirements. Its not all just give by the mother and take by the fetus, there is some return given back.

This question also plays into 'political' thinking and the 'pro-choice' vs 'pro-life' stances, which no-one here should get into as none of that grandstanding has anything to do with science.