Molar pregnancy is an abnormal form of pregnancy in which a non-viable fertilized egg implants in the uterus and will fail to come to term. A molar pregnancy is a gestational trophoblastic disease^[1] which grows into a mass in the uterus that has swollen chorionic villi. These villi grow in clusters that resemble grapes.^[2] 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. It is characterized by the presence of a hydatidiform mole (or hydatid mole, mola hydatidosa).^[3] 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.^[4] The genotype is typically 46,XX (diploid) due to the subsequent mitosis of the fertilizing sperm but can also be 46,XY (diploid).^[4] 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).^[4]

Complete hydatidiform 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.^[4] 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).^[5]

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