FETUS IS NOT A BABY (GOOGLE THE HUMAN DEVELOPMENT CHART), but a parasite because the classification of the biological relationship that is based on the behavior one organism (fetus) and how it relates to the woman's body.
as a zygote, it invaded the woman's uterus using its TROPHOBLAST cells, hijacked her immune system by using NEUROKININ B and HCG--- so her body doesn't kill it, steals her nutrients to survive, and causes her harm or potential death.
"The placenta functions as an immunological barrier between the mother and the fetus, creating an immunologically privileged site. For this purpose, it uses several mechanisms:
It secretes Neurokinin B containing phosphocholine molecules. This is the same mechanism used by parasitic nematodes to avoid detection by the immune system of their host."
"Progesterone enriches the uterus with a thick lining of blood vessels and capillaries so that it can sustain the growing fetus. Due to its highly-negative charge, hCG may repel the immune cells of the mother, protecting the fetus during the first trimester. It has also been hypothesized that hCG may be a placental link for the development of local maternal immunotolerance."
"It is also possible for a symbiotic relationship to exist between two organisms of the same species."
http://www.answers.com/topic/symbiosis -- Gale's Science of Everyday Things.
just like a parasitic twin --- http://en.wikipedia.org/wiki/Parasitic_twin
"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 it"
pregnancy CAUSES HARM: http://www.thelizlibrary.org/liz/004.htm