Hydatidiform mole

Molar pregnancy is an abnormal form of pregnancy, characterized by the presence of a hydatidiform mole (or hydatid mole), an anomalous growth containing a nonviable embryo which implants and proliferates within the uterus.^[1] In some cases the uterus contains a normal embryo in addition to the mole. A hydatidiform mole is removed upon diagnosis because there is some risk that it develop into choriocarcinoma, a form of cancer.

The term is derived from hydatidiform (Greek "like a bunch of grapes"), referring to the shape of the growth, and mole (from Latin mola = millstone).

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Natural history

A mole is characterized by a conceptus of hyperplastic trophoblastic tissue attached to the placenta. The conceptus does not contain the inner cell mass of a normal embryo.

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.

• A complete mole usually occurs when an empty egg is fertilized by a single sperm that then duplicates its own DNA (a process called androgenesis). The DNA of a complete mole 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 XX (female), and 10% are XY (male). Most complete moles are of the XX (female) genotype because the XY (male) genotype requires two sperms to contribute both X and Y chromosomes: in this case, the DNA duplication step is skipped. The YY genotype is not seen, presumably because the Y chromosome lacks certain genes necessary for early development. 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.

• Partial moles can occur if a normal haploid egg is fertilized by two sperms, or if fertilized by one sperm and the paternal chromosomes are duplicated. Thus their DNA is both maternal and paternal in origin. They can be triploid (e.g. 69 XXX or 69 XXY instead of 46 XX or 46 XY) or even tetraploid. Fetal parts are often seen on gross examination.

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.^[2]

Hydatifdiform 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]

Clinical presentation and diagnosis

Molar pregnancies usually present with painless vaginal bleeding in the fourth to fifth month of pregnancy.^[4] The uterus may be larger than expected, or the ovaries may be enlarged. There may also be more vomiting than would be expected (hyperemesis). Sometimes there is an increase in blood pressure along with protein in the urine. Blood tests will show very high levels of human chorionic gonadotropin (hCG).^[5]

The diagnosis is strongly suggested by ultrasound (sonogram), but definitive diagnosis requires histopathological examination. The mole grossly resembles a bunch of grapes ("cluster of grapes" or "honeycombed uterus" or "snow-storm"^[6]).

Sometimes symptoms of hyperthyroidism are seen, due to the extremely high levels of hCG, which can mimic the normal Thyroid-stimulating hormone (TSH).^[7]

Treatment

Hydatidiform moles should be treated by evacuating the uterus by uterine suction or by surgical curettage as soon as possible after diagnosis, in order to avoid the risks of choriocarcinoma.^[8] Patients are followed up until their serum human chorionic gonadotrophin (hCG) level has fallen to an undetectable level. Invasive or metastatic moles (cancer) may require chemotherapy and often respond well to methotrexate. The response to treatment is nearly 100%. Patients are advised not to conceive for one year after a molar pregnancy. The chances of having another molar pregnancy are approximately 1%.

Management is more complicated when the mole occurs together with one or more normal fetuses.

Prognosis

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.

References

See also

• Gestational trophoblastic disease