Hydatidiform Moles in Pregnancy Content from the guide to life, the universe and everything

Hydatidiform Moles in Pregnancy

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A Hydatidiform Mole (Molar Pregnancy) occurs during fertilisation. There seems to be no definite reason why, although there are many theories. A mole is made up of placental tissue that has grown dramatically more than normal, forming an hydatid1 or cyst.

There are two types of moles:

  • Complete mole
    A complete mole rarely has any foetal development, and if so, very little.

  • Incomplete (partial) mole
    In this scenario, there is usually the presence of a foetus and umbilical cord. The foetus however will not usually last beyond the ninth week following the last menstrual period. A partial mole is less common than a complete mole.

What are the Chances?

For unknown reasons there are significant differences in the incidence of Hydatiform Moles from country to country. In Japan, Korea, China and the Philippines the incidence is about 1 in 550 to 600 pregnancies. In the UK this drops to about 1 in 1000, and can drop as low as 1 in 2000 in the United States. In Russia however, it can be as high as 1 in 20. Age can also increase the risk of developing a molar pregnancy. Pregnant women over the age of 50 stand 5 times more, or younger than 20, 1.5 times more of a chance of developing a mole. A previous miscarriage doubles the chance of developing a mole, and after a prior mole the risk is 30 times higher(!).


Vaginal bleeding, larger than normal growth of the uterus, no foetal movement or tissue resembling a cluster of grapes passing from the vagina can all be an indication of a possible molar pregnancy.

Invasive Moles

Although a mole is normally benign, it can persist and invade the wall of the uterus. Severe bleeding may occur should the mole penetrate through the thickness of the uterus. It can spread to other organs, usually the vagina and lungs. A persistently elevated level of the pregnancy hormone, hCG (human chorionic gonadotropin) may be an indication that the mole has become invasive despite efforts to remove it.


Choriocarcinoma is a type of cancer which sometimes develops after a molar pregnancy. A molar pregnancy however is not the sole cause for choriocarcinoma; it only increases the chance of it developing. It can spread throughout the body, but more commonly to the lungs, cervix, vagina, vulva, brain, liver, kidney and the gastrointestinal tract.

Gestational Trophoblastic Tumours (GTT)

GTT can follow both normal and abnormal pregnancies. The chance of developing GTT is 1000 times higher after a complete mole than a normal full term pregnancy. The chance of GTT following a complete mole is approximately 8%, and following a partial mole, about 0.5%. Tumours that do not respond to the drugs that have been administered, and have had to be removed surgically, are considered to be choriocarcinomas.

Placental Site Trophoblastic Tumours

These tumours are extremely rare, and unlike invasive moles, will usually not invade further than the wall of the uterus.


The Doctor will perform a dilation and curettage under general anesthesia. As moles cause a dramatic increase in the hCG hormone level, blood tests will be performed regularly for up to, in some cases, two years after the mole has been removed to ensure that it has not regrown or invaded the wall of the uterus. An increase in these levels could also be an indication of choriocarcinoma. In the case of an invasive mole or choriocarcinoma chemotherapy may be needed. Should this not be successful, an hysterectomy could be performed. Chemotherapy would be the normal treatment for GTT. Placental Site Trophoblastic tumours are the least likely of these conditions to respond to chemotherapy, and will usually require a hysterectomy.

Future Pregnancies

Pregnancy is possible, even after an invasive mole or choriocarcinoma. It will however be required that you avoid falling pregnant as long as the hCG levels are being monitored.

1Hydatid's are watery cysts and look very much like grapes.

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