A parasite is not a symbiotic life form. Symbiotic lifeforms work in conjunction for mutual benefit. The bacteria in your gut are symbiotic. They get a free meal, and they help you digest food. So both the host and the passenger are doing better by working together. A parasite works to its own good, but at the cost of the host. Ticks and Fleas are parasites they drain nourishment from your blood that in turn is detrimental to your health.
A fetus can actually be both a Symbiotic life form or a Parasite. With no invocation of semantics, and with complete dispassionate objectivity. The purpose of having a child is the enactment of the so called biological imperative. By having the child the parent passes their genes on into the next generation fulfilling their primary function. Living things live to reproduce. Without that we would not have living things.
That said if a child is aborted, dies during birth, kills the parent in birth, or causes the parent to die due to the drain on resources. Then the fetus can be viewed as a parasite. Potential isn't even in the equation. The life form has only resulted in damage or destruction to the host which has not benefited in any way.
However it can be said with this as with most things its a matter of relativity. The person who is pregnant is the one that decides whether the fetus is something desired, or something that is not desired. They must run the cost benefit analysis. So the definition depends entirely upon the point of view of the individuals involved.







