A molar pregnancy happens when the fertilisation of the egg by the sperm goes wrong. This leads to the growth of abnormal cells or clusters of water filled sacs inside the womb.
This condition is one of a group of conditions known as gestational trophoblastic disease (GTDs). Molar pregnancies used to be called hydatidiform moles but now most people call them molar pregnancies.
Most molar pregnancies are mostly benign (not cancerous). They are rare but they are the most common type of gestational trophoblastic tumour. In the UK, about 1 in 590 pregnancies is a molar pregnancy. In Asian women molar pregnancies are about twice as common as in Caucasian women.
Molar pregnancies can be complete or partial.
Complete molar pregnancy
If you have a complete mole, no parts of a baby (foetal tissue) form. There is only molar tissue in the womb. A complete mole happens when a sperm from the father fertilises an empty egg that contains no genes from the mother.
You have surgery to remove the molar tissue. Afterwards, in around 15 out of 100 women (around 15%) some molar tissue remains in the deeper tissues of the womb or other parts of the body. This is called a persistent gestational tumour. These women need to have chemotherapy, which can completely get rid of the abnormal cells.
Partial molar pregnancy
If you have a partial mole some foetal tissue might be seen within the molar tissue. It is important to understand that the foetal tissue cannot develop into a baby, although on an ultrasound scan it may look like a foetus.
A partial mole forms when two sperm fertilise the egg at the same time, so there is one set of chromosomes from the mother and two sets from the father.
You need surgery to remove the molar tissue. Most women with a partial mole don't need to have any further treatment. But around 1 in 100 women (around 1%) have some remaining abnormal cells in the deeper tissues of the womb or other parts of the body. This is called a persistent gestational tumour. In this case you need chemotherapy, which can completely get rid of the abnormal cells.
Very rarely a twin pregnancy will show a normal developing baby and a molar pregnancy at the same time. The risk of developing persistent trophoblastic disease is higher in this situation.
For many women it is possible for the pregnancy to continue. However if you have complications you may not be able to go ahead with the pregnancy. This is obviously a very difficult situation. Your doctor will tell you about the risks involved. They can refer you to one of the national molar pregnancy treatment centres for advice.
You might also have counselling so that you can think through any choices you might have before you make a decision.
Persistent trophoblastic disease
Persistent trophoblastic disease is when you have had treatment to remove a molar pregnancy but still have some molar tissue left and it grows and forms a tumour. If it is not treated it can grow quickly and cause a number of medical problems. Most women with persistent trophoblastic disease need to have chemotherapy treatment.