parasitism involves a parasite and a host that are two different species, and in which the parasite gains at the expense of the host. While a fetus is gaining physiologically at the expense of the mother, it is a single-species arrangement and the mother is gaining from an evolutionary basis by propogating genetic information into an offspring. So, it is not even an asymmetric relation like parasitism. No, a fetus (of the same species as the parent) is not a parasite.