A hydatidiform mole is a growing mass of tissue inside your womb (uterus) that will not develop into a baby. It is the result of abnormal conception. It may cause bleeding in early pregnancy and is usually picked up in an early pregnancy ultrasound scan. It needs to be removed and most women can expect a full recovery. However, close follow-up is needed after a hydatidiform mole because there is a small chance of developing a type of cancer. If a cancer does develop, effective treatment is available and most women can be cured.
Chromosomes are found in the centre (the nucleus) of the cells in your body. They carry genetic information in the form of genes. Most of the cells in your body contain 46 chromosomes arranged in 23 pairs. One chromosome from each pair is inherited from your mother and the other is inherited from your father. These 46 chromosomes make up your genetic code. This code determines what you look like, how your body functions, whether you are male or female and sometimes even whether you will develop certain diseases.
However, the reproductive cells - eggs (ova) in women and sperm in men - only have 23 chromosomes. This is so that, when a sperm fertilises an egg during normal conception, the child that is produced has 46 chromosomes in each cell - 23 from their mother and 23 from their father. The fertilised egg (ovum) has a complete set of genetic material.
After fertilisation, some cells from the fertilised ovum (called the trophoblast cells) develop into the placenta and membranes that form around the developing baby. The other cells develop into the embryo from which the baby grows. As the placenta develops, the trophoblast cells grow into and attach to the lining of the womb (uterus), allowing the pregnancy to implant in the uterus.
What is a hydatidiform mole?
A hydatidiform mole is a growing mass of tissue inside the womb (uterus) that will not develop into a baby. It is the result of abnormal conception. A hydatidiform mole may be either complete or partial.
Sometimes, during conception, a sperm fertilises an 'empty' egg (ovum). This is an ovum not carrying any chromosomes or genetic material. Under normal circumstances, the fertilised empty ovum would die and not implant in the uterus. But rarely, the ovum doesn't die and implantation takes place. The trophoblast cells grow and develop as a disorganised mass of tissue but the embryo does not develop. This is a complete hydatidiform mole. There is no tissue resembling an unborn baby (a fetus) at all.
A partial hydatidiform mole is also possible if conception doesn't take place normally. It is usually due to two sperm fertilising one normal ovum (which should not usually happen). This means that there is too much genetic material present. There is also too much trophoblastic tissue. The growth of the trophoblastic tissue overtakes the growth of any fetal tissue and the fetus does not develop normally.
Partial and complete hydatidiform moles will not produce a live baby. (They are 'non-viable' pregnancies.) A pregnancy that results in a hydatidiform mole is called a molar pregnancy.
A hydatidiform mole is part of a group of disorders called gestational trophoblastic disease (GTD). Partial and complete hydatidiform moles are benign (not cancerous) forms of GTD. However, there is a risk that a hydatidiform mole can develop into a cancerous (malignant) type of GTD, known as gestational trophoblastic neoplasia (GTN). This is described below.
How common is a hydatidiform mole?
Hydatidiform mole is rare in the UK. There is about 1 molar pregnancy for every 714 live births. This works out to be fewer than 1,000 hydatidiform moles diagnosed per year in the UK.
Any woman of childbearing age can develop a molar pregnancy but women who are aged under 16 and over 45 have a higher risk. For some reason, women of Asian backgrounds are also more likely to have a molar pregnancy.
What are the symptoms of a hydatidiform mole?
Women with a hydatidiform mole usually have higher-than-average levels of the pregnancy hormone human chorionic gonadotrophin (hCG) compared with women with a normal pregnancy. This hormone is produced by the trophoblastic tissue. It is the hormone that is detected in a standard pregnancy test. The high levels of hCG occur because there is an excessive amount of trophoblastic tissue with a hydatidiform mole. The high hCG levels are responsible for some of the symptoms.
Pregnancy symptoms. You may have signs of pregnancy, including your periods stopping, feeling sick (nausea), being sick (vomiting), breast tenderness, etc. You may notice that you feel bigger than expected for the number of weeks you are pregnant. This is because a molar pregnancy grows more quickly than a normal pregnancy would, due to the abnormally developing trophoblastic tissue.
Bleeding. You may have vaginal bleeding early in the pregnancy. This is the most common symptom. Many women suspect that they are having a miscarriage.
No symptoms. Some women with a hydatidiform mole have no symptoms. The molar pregnancy is diagnosed after a routine pregnancy ultrasound scan (see below).
Rare symptoms. Very rarely, you may have problems with high blood pressure, very bad nausea and vomiting (hyperemesis gravidarum) or symptoms of an overactive thyroid gland may develop.
How is a hydatidiform mole diagnosed?
If you develop bleeding in early pregnancy, your doctor will usually arrange an ultrasound scan. Sometimes, the picture on the ultrasound scan can be quite characteristic if you have a hydatidiform mole. However, at other times the ultrasound picture may just look like a miscarriage. If your doctor suspects that you have a hydatidiform mole, they may also suggest a blood test to check your levels of hCG. Higher-than-normal levels can help with the diagnosis.
If you have an ultrasound picture that looks like a miscarriage and higher-than-normal hCG levels, the definite diagnosis of hydatidiform mole is made when the pregnancy is removed and examined under a microscope (see below).
If you do not have bleeding or other symptoms, the diagnosis of hydatidiform mole is usually suspected when you have a routine pregnancy ultrasound scan. The further through the pregnancy you are, the more characteristic the ultrasound picture of a molar pregnancy becomes.
What is the treatment for a hydatidiform mole?
If you have a hydatidiform mole, you will need to have it removed. This means having a small operation. This is done in hospital by a doctor who is a gynaecology specialist. You will be given an anaesthetic. In most cases, a small tube is passed into your womb (uterus) through the opening of your uterus (your cervix) and the abnormal tissue is removed by suction. The tissue is then sent off to the laboratory for examination under the microscope.
What follow-up do I need after a hydatidiform mole?
As mentioned above, there is a small risk that you can develop GTN - the cancerous form of the condition - after a hydatidiform mole. There is a risk that this can spread (metastasise) to other parts of your body including your lung, liver or brain. Therefore, you need close follow-up after a hydatidiform mole to monitor for this.
When you are first diagnosed with a hydatidiform mole, your hCG level will be raised. When the hydatidiform mole is treated (removed), the hCG level will usually return to a normal, non-pregnant amount and should remain so. If you develop GTN, the hCG level can remain elevated or continue to rise further. So, this blood test is a good way to check for the possible development of GTN.
A hydatidiform mole is relatively rare in the UK. Therefore, if you are diagnosed as having this condition, it is very important to be registered with one of the three specialist centres (located in either Dundee, Sheffield or London). Your gynaecologist should arrange this for you. These centres will arrange follow-up blood/urine tests for hCG to look for any evidence of GTN developing. They will advise you on how often to have the tests. Follow-up will be for at least six months after your hCG levels have returned to normal provided you do not develop GTN.
Is treatment possible if I develop gestational trophoblastic neoplasia?
Yes. Treatment is possible and is effective in most cases.
About 1 in 200 women with partial moles and 15 in 100 women with complete moles will need treatment because hCG levels are not returning to normal. The exact treatment given depends on things such as your hCG levels, whether there is spread to other parts of the body, etc. Treatment involves injections of different medications (chemotherapy) such as methotrexate, etoposide and dactinomycin (actinomycin D). Treatment is continued until your hCG level has returned to normal and then for some weeks afterwards.
What is the outlook (prognosis)?
Most women who have a hydatidiform mole have an excellent outlook. As mentioned above, about 1 in 200 women with partial moles and 15 in 100 women with complete moles will need treatment for GTN. In those who develop GTN, at least 98 in 100 are completely cured.
When is it safe to try for another baby?
Your specialist will advise you when it is safe to try for another baby if you have had a hydatidiform mole or GTN. Usually you will be advised not to become pregnant until your hCG tests have been normal for six months after a molar pregnancy, and for a year after finishing chemotherapy. You can use most types of contraception without having to wait for your tests to return to normal. This includes hormone contraception, such as the combined oral contraceptive pill or progestogen-only contraceptive pill, if there are no factors which make them unsuitable for you personally.
It is then very unlikely that you would have another hydatidiform mole if you became pregnant again. Having had one hydatidiform mole makes it more likely that you can have another but it is still very unusual. Fewer than 2 in 100 women who fall pregnant after previously having a hydatidiform mole will develop another hydatidiform mole in a subsequent pregnancy.
Rarely, GTD can be 'reactivated' after a future pregnancy even several years later. For this reason, your hCG levels should be monitored for a time after any pregnancy ends (be it a normal pregnancy, a miscarriage, a termination, etc). You or your gynaecologist should contact the specialist centre that you are registered with to arrange this.
Further reading and references
; Royal College of Obstetricians and Gynaecologists (February 2010)
; European Society of Medical Oncology (Sept 2013)
; Gestational trophoblastic diseases - clinical guidelines for diagnosis, treatment, follow-up, and counselling. Dan Med J. 2015 Nov62(11):A5082.
; Update on the diagnosis and management of gestational trophoblastic disease. Int J Gynaecol Obstet. 2015 Oct131 Suppl 2:S123-6. doi: 10.1016/j.ijgo.2015.06.008.
; National Cancer Institute (US)
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