Hydatidiform mole is a condition where there is abnormal growth and degeneration of placental tissue to produce a large number of fluid filled sacks (the word comes from the Greek for water) which look like a bunch of grapes. It usually arises as a result of sperms fertilising an egg that has lost its nucleus, in which case there is no foetus present, or alternatively as a result of two sperms entering an egg to produce an abnormal foetus with three sets of chromosomes, in which case it is called a partial mole. Because there is an excess of placental tissue, there is an excess of the hormone it produces (human chorionic gonadotrophin) and so there are excessive symptoms of pregnancy. This is a condition that can predispose to the development of malignant disease called chorionic carcinoma (described in this section) and therefore needs careful monitoring.