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. It falls under the category of gestational trophoblastic diseases and was previously known as a hydatidiform mole. During a molar pregnancy, the uterus contains a growing mass characterized by swollen chorionic villi, resembling clusters of grapes. The occurrence of a molar pregnancy can be attributed to the fertilized egg lacking an original maternal nucleus. As a result, the products of conception may or may not contain fetal tissue. These molar pregnancies are categorized into two types: partial moles and complete moles, where the term 'mole' simply denotes a clump of growing tissue or a ‘growth'.
A complete mole is caused by either a single sperm (90% of the time) or two sperm (10% of the time) combining with an egg that has lost its DNA. In the former case, the sperm reduplicates, leading to the formation of a "complete" 46-chromosome set. Typically, the genotype is 46,XX (diploid) due to subsequent mitosis of the fertilizing sperm, but it can also be 46,XY (diploid). However, 46,YY (diploid) is not observed. On the other hand, a partial mole occurs when a normal egg is fertilized by one or two sperm, which then reduplicates itself, resulting in genotypes of 69,XXY (triploid) or 92,XXXY (tetraploid).
Complete moles carry a 2–4% risk, in Western countries, of developing into choriocarcinoma and a higher risk of 10–15% in Eastern countries, with an additional 15% risk of becoming an invasive mole. In contrast, incomplete moles can become invasive as well but are not associated with choriocarcinoma. Notably, complete hydatidiform moles account for 50% of all cases of choriocarcinoma.
Molar pregnancies are relatively rare complications of pregnancy, occurring in approximately 1 in 1,000 pregnancies in the United States, while in Asia, the rates are considerably higher, reaching up to 1 in 100 pregnancies in countries like Indonesia.