Light micrograph of a section through a human female uterus showing a chorionic carcinoma. This is a rare type of tumour that happens in women during pregnancy. It is described as a fast-growing malignant tumour that develops from trophoblastic cells (cells that help an embryo attach to the uterus and help form the placenta). Almost all chorionic carcinomas form in the uterus after fertilisation of an egg by a sperm, but a small number form in a testis or an ovary.