To call a zygote/embryo/fetus a parasite is wrong. If you are to look up the word you’ll see that(see below). Definition one, a parasite has to be a different species** that its host. Definition two, you would have to admit the “parasite” is a person.
par⋅a⋅site /ˈpærəˌsaɪt/ [par-uh-sahyt]
–noun 1. an organism that lives on or in an organism of another species, known as the host, from the body of which it obtains nutriment.
2. a person who receives support, advantage, or the like, from another or others without giving any useful or proper return, as one who lives on the hospitality of others.
3. (in ancient Greece) a person who received free meals in return for amusing or impudent conversation, flattering remarks, etc.
Origin:
1530–40; < L parasītus < Gk parásītos one who eats at another’s table, orig. adj.: feeding beside, equiv. to para- para- 1 + sît(os) grain, food + -os adj. suffix