Human chorionic gonaotropin or hCG is a hormone produced by the placenta during pregnancy.
hCG has several functions, including promoting the production of progesterone and preventing the mother's immune system from attacking the fetal tissue.
Pregnancy tests usually test for the presence of hCG in the blood or urine. However, some tumors also produce the hormone and in a non-pregnant patient high hCG levels point to cancer.
In Dead & Buried, the doctors are misled by Iris's positive pregnancy tests into thinking she is pregnant when in fact she has Choriocarcinoma.
Human chorionic gonadotropin at Wikipedia