system
Given the following post, respond as if you're commenting on Reddit:
human
If a pregnant woman has cancer, is it possible for the cancer to spread to the fetus?
gpt
It’s exceptionally rare.  Metastatic disease is usually caused by seeding through the blood supply, or, invasive growth into new organs. 

Because mums and bubs don’t actually share a blood supply (their supplies run very close together in the placenta, facilitating transfer of nutrients etc, it’s very hard to seed from one to the another, and invasive growth is pretty damn hard through amniotic fluid. 

There’s a couple of dozen of “proven” cases, mostly leukaemia and melanoma (while melanoma is a solid tumour, it’s insanely invasive), and leukaemia is a liquid tumour which would facilitate the transfer from mum to bubs blood.