Parasites are 1) almost always an organism from another species that invades the body of an organism from a different species. 2) Parasites are near always detrimental to the health of the host. The only one I can think of that is not "that bad" is the tongue eating louse, and i) I reckon most people don't want to lose their tongues ii) iirc there are studies that show that fish infected with them are actually underweight so there you go. Fetuses are not from a different species and are not invasive as they come from within the body.
The relationship between a fetuses and its mother is closer to mutualism or commensalism.





