Molar pregnancy is an abnormal form of pregnancy in which a non-viable fertilized egg implants in the uterus and converts a normal pregnancy into an abnormal one (which will fail to come to term). A molar pregnancy is a gestational trophoblastic disease that grows into a mass in the uterus that has swollen chorionic villi. These villi grow in clusters that resemble grapes. A molar pregnancy can develop when an egg that is missing its nucleus is fertilized and that may or may not contain fetal tissue. It is characterized by the presence of a hydatidiform mole (or hydatid mole, mola hydatidosa). Molar pregnancies are categorized into partial and complete moles. Mole as used here simply indicates clump of growing tissue, or a 'growth'. A complete mole is caused by a single (90%) or two (10%) sperm combining with an egg which has lost its DNA (the sperm then reduplicates forming a "complete" 46 chromosome set)  The genotype is typically 46,XX (diploid) due to subsequent mitosis of the fertilizing sperm, but can also be 46,XY (diploid). In contrast, a partial mole occurs when an egg is fertilized by two sperm or by one sperm which reduplicates itself yielding the genotypes of 69,XXY (triploid) or 92,XXXY (tetraploid). Complete hydatidiform moles have a higher risk of developing into choriocarcinoma — a malignant tumor of trophoblast cells — than do partial moles. The etymology is derived from hydatisia (Greek "a drop of water"), referring to the watery contents of the cysts, and mole (from Latin mola = millstone/false conception). The term, however, comes from the similar appearance of the cyst to a hydatid cyst in an Echinococcosis. A hydatidiform mole conception may be categorized in medical terms as one type of non-induced (natural) "missed abortion" - referred to colloquially as a "missed miscarriage", because the pregnancy has become non-viable (miscarried) but was not immediately expelled (therefore was "missed").