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Molar pregnancy is an abnormal form of pregnancy, characterized by the presence of a hydatidiform mole (or hydatid mole, mola hytadidosa), an anomalous growth containing a nonviable embryo which implants and proliferates within the uterus.[1] A hydatidiform mole is removed upon diagnosis because there is some risk that it may develop into choriocarcinoma, a form of cancer.
The etymology is derived from hydatis (Greek “a drop of water”), referring to the watery contents of the cysts, and mole (from Latin mola = millstone/false conception).[2] The term, however, comes from the cyst’s appearance looking very much like a hydatid cyst in an Echinococcosis.
A hydatidiform mole is a pregnancy/conceptus in which the placenta contains grapelike vesicles that are usually visible with the naked eye. The vesicles arise by distention of the chorionic villi by fluid. When inspected in the microscope, hyperplasia of the trophoblastic tissue is noted. If left untreated, a hydatidiform mole will almost always end as a spontaneous abortion.
Based on morphology, hydatidiform moles can be divided into two types: In complete moles, all the chorionic villi are vesicular, and no sign of embryonic or fetal development is present. In partial moles some villi are vesicular, whereas others appear more normal, and embryonic/fetal development may be seen but the fetus is always malformed and is never viable.
Hydatidiform moles are a common complication of pregnancy, occurring once in every 1000 pregnancies in the US, with much higher rates in Asia (e.g. up to one in 100 pregnancies in Indonesia).[3]
In rare cases a hydatidiform mole co-exists in the uterus with a normal, viable fetus. These cases are due to twinning. The uterus contains two conceptuses: one with an abnormal placenta and no viable fetus (the mole), and one with a normal placenta and a viable fetus. Under careful surveillance it is often possible for the woman to give birth to the normal child and to be cured of the mole.[4]
The etiology 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 or over 40 years of age have a higher risk. Other risk factors include diets low in protein, folic acid, and carotene.[5] The diploid set of sperm-only DNA means that all chromosomes have sperm-patterned methylation suppression of genes. This leads to overgrowth of the syncytiotrophoblast whereas dual egg-patterned methylation leads to a devotion of resources to the embryo, with an underdeveloped syncytiotrophoblast. This is considered to be the result of evolutionary competition with male genes driving for high investment into the fetus versus female genes driving for resource restriction to maximise the number of children.[6]
In most hydatidiform moles, the parental origin of the genes in the cellular nucleus is abnormal.
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