Here are several definitions of PARASITE taken from the same dictionary website...http://www.thefreedictionary.com/parasite

~Biology- An organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host....I suppose an unborn baby fits this definition...unless different organism means as in a different species or something like that. Either way, it doesn't seem like a derogatory comparison

~parasite [ˈpӕrəsait]-an animal or plant that lives on another animal or plant without giving anything in return
No problem here, not derogatory either


~ (Life Sciences & Allied Applications / Biology) an animal or plant that lives in or on another (the host) from which it obtains nourishment. The host does not benefit from the association and is often harmed by itDoesn't fit this definition for couple of reasons...the host is not often harmed by it and the host, mother, will definitely be benefitted by it with the joy of a brand new baby!