Molar pregnancy

Molar pregnancy
Other names	Hydatid mole, hydatidiform mole
Histopathologic image of hydatidiform mole (complete type). H&E stain.
Specialty	Obstetrics

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.^[1] During a molar pregnancy, the uterus contains a growing mass characterized by swollen chorionic villi, resembling clusters of grapes.^[2] 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.^[3] Typically, the genotype is 46,XX (diploid) due to subsequent mitosis of the fertilizing sperm, but it can also be 46,XY (diploid).^[3] 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).^[3]

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.^[3] 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.^[4]