Molar pregnancy is an abnormal form of pregnancy, wherein a non-viable, fertilized egg implants in the uterus, and thereby converts normal pregnancy processes into pathological ones. It is characterized by the presence of a hydatidiform mole (or hydatid mole, mola hydatidosa). Molar pregnancies are categorized into partial and complete moles. Complete moles have no identifiable embryonic or fetal tissues and arise when an empty egg with no nucleus is fertilized by one (or occasionally two) normal sperm; the genotype is typically 46,XX (due to subsequent mitosis of the fertilizing sperm) but can also be 46,XY or 23,X. In contrast, a partial mole occurs when a normal egg is fertilized by two (or occasionally three) spermatozoa, and the genotypes can be 69,XXY or 92,XXXY. Hydatidiform moles may develop into choriocarcinoma, a form of cancer.