Light micrograph of a section of human tissue showing gestational trophoblastic disease (GTD). This is a group of rare diseases in which abnormal trophoblastic cells grow inside the uterus after conception. In GTD, a tumour develops inside the uterus from tissue that forms after conception (the joining of sperm and egg). The most common types of gestational trophoblastic disease occur when a sperm cell fertilises and empty egg cell or when two sperm cells fertilise a normal egg cell. The risk of this disease is higher in older women.