Choriocarcinoma is an oddment disease of pregnancy. Choriocarcinoma occurs when the invasive system used for placental invasion or pregnancy implantation goes out of control. If a mother produces an extreme excess of hyperglycosylated hCG in the third trimester of pregnancy, it can drive the small plug of placental tissue remaining in the uterus after delivery of the baby and placenta, the extravillous cytotrophoblast cells, to grow and invade the body. Choriocarcinoma can invade the whole body with cancer-like metastases in the lung and brain. Choriocarcinoma can also be limited in some cases to the uterus and uterus and pelvis.
Choriocarcinoma is not really a cancer, it is extravillous cytotrophoblast cells invading the body in a manner reminiscent of cancer but actually is just the normal placental invasion system gone out of control. This is driven by the autocrine that normally drive the placental invasive system or hyperglycosylated hCG.
Physicians only call the disease choriocarcinoma when a surgical sample is obtained and histology has confirmed the identity of the cells, until this point it is called gestational trophoblastic neoplasm. Until surgery is perfomed histology is not possible. As such some choriocarcinoma-like cases are sometimes permanently called gestational trophoblastic neoplasm.