A molar pregnancy also known as a hydatidiform mole, is an abnormal form of pregnancy in which a non-viable fertilized egg implants in the uterus. A molar pregnancy is a type of gestational trophoblastic disease that used to be known as a hydatidiform mole. A molar pregnancy grows into a mass in the uterus that has swollen chorionic villi that grow in clusters resembling grapes. A molar pregnancy can develop when a fertilized egg does not contain an original maternal nucleus. The products of conception may or may not contain fetal tissue. Molar pregnancies are categorized as partial moles or complete moles, with the word 'mole' being used to denote simply a clump of growing tissue, or a 'growth'. A complete mole is caused by a single sperm (90% of the time) or two (10% of the time) sperm combining with an egg which has lost its DNA. In the first case, the sperm then reduplicates, forming a "complete" 46 chromosome set. The genotype is typically 46,XX (diploid) due to the subsequent mitosis of the fertilizing sperm but can also be 46,XY (diploid). 46,YY (diploid) is not observed. In contrast, a partial mole occurs when a normal egg is fertilized by one or two sperm which then reduplicates itself, yielding the genotypes of 69,XXY (triploid) or 92,XXXY (tetraploid). Complete moles have a 2–4% risk of developing into choriocarcinoma in Western countries and 10–15% in Eastern countries and a 15% risk of becoming an invasive mole. Incomplete moles can become invasive (<5% risk) but are not associated with choriocarcinoma. Complete hydatidiform moles account for 50% of all cases of choriocarcinoma. Molar pregnancies are a relatively rare complication of pregnancy, making up 1 in 1,000 pregnancies in the US, with much higher rates in Asia (e.g. up to 1 in 100 pregnancies in Indonesia). Signs and symptoms Molar pregnancies usually present with painless vaginal bleeding in the fourth to fifth months of pregnancy. The uterus may be larger than expected, or the ovaries may be enlarged. There may also be more vomiting than would be expected (hyperemesis). Sometimes there is an increase in blood pressure along with protein in the urine. Blood tests will show very high levels of human chorionic gonadotropin (hCG). Cause The cause of this condition is not completely understood. Potential risk factors may include defects in the egg, abnormalities within the uterus, or nutritional deficiencies. Women under 20