Hydatidiform mole Dr Rob Hicks The condition affects as many as one in every 700-800 pregnancies in the UK.

In this article

What is a hydatidiform mole? Symptoms

Causes and risk factors Treatment and recovery

A hydatiform mole is one condition in a range of problems known as trophoblastic disease, where a pregnancy doesn’t grow as it should. It’s also often called a molar pregnancy.

There are two different types of molar pregnancy, which differ in how they form and how they need to be treated.

In a normal pregnancy genetic material from both the mother and the father combine at fertilization to form the new life. But in a molar pregnancy this process goes wrong. In what is known as a complete molar pregnancy, the maternal chromosomes are lost (either at the time of conception or earlier while the egg was forming in the ovary) and there is only genetic material from the father in the cells that develop. In a partial molar pregnancy, there is a set of maternal chromosomes but also two sets of chromosomes from the father (ie double the normal paternal genetic material).

Complete molar pregnancies develop as a mass of rapidly growing cells but without a foetus – it cannot therefore develop into a baby. It used to be described as looking rather like a huge bunch of grapes, but these days most cases are identified on ultrasound and treated long before they reach this stage. In a partial molar pregnancy a foetus may initially develop but because of the imbalance in genetic material from both the father and the mother, the foetus is always abnormal and can not survive beyond the first 3 months of the pregnancy.

Hydatidiform moles are often harmless but if untreated can keep on growing and become invasive, spreading to the organs around it, or even further afield to the lungs, liver or brain. Very rarely, in 2-3% of cases, it may become malignant. These cancerous types of trophoblastic disease are called choriocarcinoma and placental site trophoblast tumours.

A woman with a hydatidiform mole often feels pregnant and has symptoms such as morning sickness, probably because the cells of the molar pregnancy produce the pregnancy hormone hCG (human chorionic gonadotrophin). This is also the hormone that is used in a pregnancy test, so she may have a positive result. Some women have no pregnancy symptoms (as with many normal pregnancies).

However, the mole grows faster than a normal foetus would, so the abdomen may become larger more quickly than would be expected for the dates of the pregnancy. The woman may experience abdominal pain, and also severe nausea and vomiting (hyperemesis).

Bleeding from the vagina is another common warning symptom that things are not as they should be. Symptoms similar to pre-eclampsia - high blood pressure, protein in the urine, swelling of the feet and legs - may also occur in the first trimester or early in the second.

Most molar pregnancies are diagnosed at the first ultrasound scan that shows a mass of cells without the presence of a foetus in a complete molar pregnancy, or an abnormal non-viable foetus and placenta in a partial mole.

It remains unclear why a hydatidiform mole develops. However, there are a number of possible reasons, including defects in the egg, maternal nutritional deficiencies and uterine abnormalities. Women under 20 or over 40 are at higher risk.

Having a diet that's low in protein, folic acid and carotene also increases the risk of a hydatidiform mole. The number of times a women has been pregnant, however, doesn't influence her risk.

Once it has been established that a woman is carrying a hydatidiform mole rather than a healthy foetus, suction evacuation is used to remove the pregnancy from the womb. This is curative in about 4 out of 5 molar pregnancies.

It is then important to monitor the woman’s progress and repeatedly measure human chorionic gonadotropin (hCG) to be sure that everything settles back down to a normal, non-pregnancy level.

About 15% of women who have had a complete molar pregnancy and 0.5% of those with a partial molar pregnancy will require additional treatment usually either because hCG levels hit a plateau or start to rise again, or because of persistent heavy vaginal bleeding.

Further treatment may involve the use of chemotherapy (usually a drug called Methotrexate combined with folinic acid), esecially if there is any concern about invasive or malignant disease.

More than 99% of hydatidiform moles are cured, and even the more aggressive choriocarcinoma has a cure rate over 90%.

Following successful treatment, most women can have children if they wish. However, it's strongly recommended that a woman who has had a molar pregnancy doesn't become pregnant again for 12 months. Although the likelihood is small, there's a real risk of malignant disease developing and the increase in pregnancy hormones this would cause can't be distinguished from those of a real pregnancy. Consequently, good contraception is required, as is regular monitoring by a hospital specialist.

This article was last medically reviewed by Dr Trisha Macnair in March 2009.