Molar pregnancy

What is a molar pregnancy?

A molar pregnancy is one condition in a range of problems known as trophoblastic disease, where a pregnancy doesn’t grow as it should. It's sometimes called a hydatiform mole.
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 the mother and father combines to form new life. In a molar pregnancy, this process goes wrong. In a complete molar pregnancy, the maternal chromosomes are lost, either at conception or while the egg was forming in the ovary, and only genetic material from the father develops in the cells. 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.
In a partial molar pregnancy, a foetus may start to develop but because of the imbalance in genetic material, it's always abnormal and can't survive beyond the first three months of pregnancy.
A molar pregnancy is 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 two to three per cent of cases, it may become malignant. These cancerous types of trophoblastic disease are called choriocarcinoma and placental site trophoblast tumours.

Symptoms of molar pregnancy

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).
As the mole grows faster than a normal foetus would, 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 sign 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, which 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.

Causes of molar pregnancy

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 molar pregnancy. The number of times a women has been pregnant, however, doesn't influence her risk.

Treatment of molar pregnancy

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 four out of five molar pregnancies.
It's 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 per cent of women who have had a complete molar pregnancy and 0.5 per cent of those with a partial molar pregnancy will require additional treatment, 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 methotrexate combined with folinic acid), especially if there's any concern about invasive or malignant disease.
More than 99 per cent of hydatidiform moles are cured, and even the more aggressive choriocarcinoma has a cure rate over 90 per cent.

Subsequent pregnancies

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.
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