Hydatiform mole

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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Hydatidiform mole (or mola hydatidiforma) is 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). It consists of a nonviable embryo which implants and proliferates within the uterus.^[1] The term is derived from hydatidiform (“like a bunch of grapes”) and mole (from Latin mola = millstone).

Most moles present with painless vaginal bleeding during the second trimester of pregnancy. They are diagnosed by ultrasound imaging. Extremely high levels of human chorionic gonadotropin (HCG) are suggestive, but not diagnostic, of molar pregnancy.^[2] Today moles are surgically removed by curettage, in order to avoid the risks of choriocarcinoma.^[3]

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

A mole is characterized by a conceptus of hyperplastic trophoblastic tissue attached to the placenta. The conceptus does not contain the inner cell mass (the mass of cells inside the primordial embryo that will eventually give rise to the fetus).

The hydatidiform mole can be of two types: a complete mole, in which the abnormal embryonic tissue is derived from the father only; and a partial mole, in which the abnormal tissue is derived from both parents.

• Complete moles usually occur when an empty ovum is fertilized by a sperm that then duplicates its own DNA (a process called androgenesis). This explains why most complete moles are of the 46,XX genotype. A 46, XY genotype may occur when 2 sperm (one 23, X and the other 23, Y) fertilize an empty egg. They grossly resemble a bunch of grapes (“cluster of grapes” or “honeycombed uterus” or “snow-storm”^[1]). Their DNA is purely paternal in origin (since all chromosomes are derived from the sperm), and is diploid (i.e. there are two copies of every chromosome). Ninety percent are 46,XX, and 10% are 46,XY. In a complete mole, the fetus fails to develop, thus on gross examination there are no signs of fetal tissue. All of the chorionic villi are enlarged. The main complication of the complete mole is a 2% chance of progression to a cancer called choriocarcinoma.
• Partial moles can occur if a normal haploid ovum is fertilized by two sperm, or, if fertilized by one sperm, if the paternal chromosomes become duplicated. Thus their DNA is both maternal and paternal in origin. They can be triploid (e.g. 69 XXX, 69 XXY) or even tetraploid. Fetal parts are often seen on gross examination. There is also an increased risk of choriocarcinoma, but the risk is lower than with the complete mole.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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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.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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More than 80% of hydatidiform moles are benign. The outcome after treatment is usually excellent. Close follow-up is essential. Highly effective means of contraception are recommended to avoid pregnancy for at least 6 to 12 months.

In 10 to 15% of cases, hydatidiform moles may develop into invasive moles. These may intrude so far into the uterine wall that hemorrhage or other complications develop. It is for this reason that a post-operative full abdominal and chest x-ray will often be requested.

In 2 to 3% of cases, hydatidiform moles may develop into choriocarcinoma, which is a malignant, rapidly-growing, and metastatic (spreading) form of cancer. Despite these factors which normally indicate a poor prognosis, the rate of cure after treatment with chemotherapy is high.

Over 90% of women with malignant, non-spreading cancer are able to survive and retain their ability to have children. In those with metastatic (spreading) cancer, remission remains at 75 to 85%, although the ability to have children is usually lost.