What is molar pregnancy?

A molar pregnancy occurs when the placenta develops abnormally, forming a mass of tissue (known as a mole) inside the uterus

A molar pregnancy is sometimes also known as a hydatidiform mole. The abnormal placental tissue is usually benign, but in some cases it can develop into more serious forms of gestational trophoblastic disease (GTD), which can include cancer.

Molar pregnancy is not common. Estimates vary, but it occurs in around one of every 1000-1500 pregnancies. [1] [2]

Benign

Not leading to cancerous growth. Not harmful.

Placenta

The organ that forms within the uterus of a pregnant woman to provide the fetus with nourishment from the blood supply of the mother.

Uterus

The hollow organ of the female reproductive system that is responsible for the development of the embryo and fetus during pregnancy. Also known as the womb.

Gestational trophoblastic disease

A group of tumours, not necessarily cancerous, that originate in the uterus, from the cells that normally develop into the placenta (the attachment of the embryo to the uterus).

Placental

Relating to the placenta, the organ that forms within the uterus of a pregnant woman to provide her unborn baby with nourishment from the blood supply.

1. Royal Women’s Hospital (Melbourne Vic). (2014) The Women’s health book: a complete guide to health & wellbeing for women of all ages. North Sydney N.S.W.: Random House Australia.

2. Gestational trophoblastic disease. Cancer Australia. Accessed August 12 2014 from

Signs and symptoms

A molar pregnancy is usually detected early in a pregnancy. It often ends in a miscarriage and is not able to form a viable pregnancy.

Symptoms include:

  • Vaginal bleeding during the first three months of pregnancy;
  • Growth of the uterus that does not correspond to the length of the pregnancy (it may be too large or too small);
  • Severe morning sickness, and;
  • No fetal heartbeat or movement, although molar pregnancy is almost always detected before this becomes an issue.

In a small number of cases, women with a molar pregnancy can also experience symptoms of:

  • Hyperthyroidism, and;
  • Pre-eclampsia, although earlier in pregnancy than it usually occurs (the first and early part of the second trimesters).

With a molar pregnancy there are usually symptoms that it is not a normal pregnancy; however, some women may have a miscarriage and not be aware that they have had a molar pregnancy unless the tissue from the miscarriage is tested.

Uterus

The hollow organ of the female reproductive system that is responsible for the development of the embryo and fetus during pregnancy. Also known as the womb.

1. Royal Women’s Hospital (Melbourne Vic). (2014) The Women’s health book: a complete guide to health & wellbeing for women of all ages. North Sydney N.S.W.: Random House Australia.

2. Gestational trophoblastic disease. Cancer Australia. Accessed August 12 2014 from

Causes

A molar pregnancy occurs because, at conception, the embryo develops with an extra set of chromosomes from the father. Why this occurs is not known.

In a normal embryo, cells contain one set of chromosomes from the mother and one set from the father. In a molar pregnancy, more than one set of chromosomes come from the father, either because the chromosomes from one sperm have been duplicated, or because two separate sperm have fertilised the mother's egg (ovum). A complete molar pregnancy occurs when the egg contains no chromosomes, whereas in a partial molar pregnancy the egg contain the normal 23 chromosomes.

This interferes with the normal development of the embryo. The part of the embryo that would have become the fetus (the developing baby) either does not develop at all, or is abnormal. The fetus is not viable and does not survive beyond the first few months of pregnancy. 

The placenta, which also develops from the outer cells of the embryo, grows more quickly than usual and forms an abnormal mass of tissue inside the uterus. It produces the hormone human chorionic gonadotropin (hCG), which causes many of the signs of pregnancy, such as morning sickness and tender breasts.

A diagram showing the formation of embryos in molar pregnancy.Types of molar pregnancy. 

Cells

The fundamental unit of life; the simplest living unit that can exist, grow, and reproduce independently. The human body is composed of trillions of cells of many kinds.

Chromosomes

A package of DNA tightly coiled inside the nucleus of a cell containing genetic material. Humans have 23 pairs of chromosomes.

Embryo

An organism in the early stages of development. An unborn human between the time of fertilisation and the eighth week of pregnancy.

Fetus

An unborn human, from the ninth week of pregnancy until birth.

Human chorionic gonadotropin

A hormone present in both men and women, but is produced in large quantities by the placenta in a pregnant woman. Some pregnancy tests analyse the level of this hormone in the body.

Placenta

The organ that forms within the uterus of a pregnant woman to provide the fetus with nourishment from the blood supply of the mother.

Sperm

The mature male sex cell that fertilises the female ovum.

Uterus

The hollow organ of the female reproductive system that is responsible for the development of the embryo and fetus during pregnancy. Also known as the womb.

1. Royal Women’s Hospital (Melbourne Vic). (2014) The Women’s health book: a complete guide to health & wellbeing for women of all ages. North Sydney N.S.W.: Random House Australia.

2. Gestational trophoblastic disease. Cancer Australia. Accessed August 12 2014 from

Types

Complete and partial

Molar pregnancies can be:

  • Complete, in which there is only abnormal placental tissue (and no fetus), and;
  • Partial, in which there is an abnormal fetus and sometimes some normal placental tissue.

The risk of developing more serious forms of gestational trophoblastic disease is slightly higher with complete molar pregnancies.

Invasive and non-invasive

In some cases, the placental tissue can grow into the wall of the uterus. This is called an invasive mole.

Gestational trophoblastic disease (GTD)

In about 10% of women who have a molar pregnancy, levels of hCG do not return to normal [1] . This is called persistent gestational trophoblastic disease (GTD). Symptoms can include vaginal bleeding and pain and swelling in the abdomen that continue after the pregnancy.

In rare cases, the abnormal placental tissue can develop over time into cancer.

Forms of cancer that can occur in gestational trophoblastic disease include:

  • Choriocarcinoma;
  • Placental-site trophoblastic tumour, and;
  • Epithelioid trophoblast tumour.

These cancers can occur following normal pregnancies as well as molar pregnancies and can spread to the rest of the body, particularly the lungs, liver and brain.

Fetus

An unborn human, from the ninth week of pregnancy until birth.

hCG

A hormone present in both men and women, but is produced in large quantities by the placenta in a pregnant woman. Some pregnancy tests analyse the level of this hormone in the body.

Uterus

The hollow organ of the female reproductive system that is responsible for the development of the embryo and fetus during pregnancy. Also known as the womb.

Gestational trophoblastic disease

A group of tumours, not necessarily cancerous, that originate in the uterus, from the cells that normally develop into the placenta (the attachment of the embryo to the uterus).

Placental

Relating to the placenta, the organ that forms within the uterus of a pregnant woman to provide her unborn baby with nourishment from the blood supply.

1. Royal Women’s Hospital (Melbourne Vic). (2014) The Women’s health book: a complete guide to health & wellbeing for women of all ages. North Sydney N.S.W.: Random House Australia.

Risk factors

Women may be at an increased risk of having a molar pregnancy if they:

Gestational trophoblastic disease

A group of tumours, not necessarily cancerous, that originate in the uterus, from the cells that normally develop into the placenta (the attachment of the embryo to the uterus).

1. Royal Women’s Hospital (Melbourne Vic). (2014) The Women’s health book: a complete guide to health & wellbeing for women of all ages. North Sydney N.S.W.: Random House Australia.

Methods for diagnosis

In some cases, a molar pregnancy will be detected as part of a routine ultrasound during early pregnancy. Your doctor will ask you about your symptoms and your medical history. They may ask questions about any previous pregnancies, births, miscarriages or abortions.

Tests may include:

Other tests such as a computerised tomography (CT) scan or magnetic resonance imaging (MRI) may be recommended if other health conditions (including more serious forms of gestational trophoblastic disease) are suspected of causing symptoms.

Abortions

The termination of a pregnancy.

Computerised tomography

A scan that uses X-rays to create a 3D image of the body. This can detect abnormalities more effectively than a simple X-ray can.

Human chorionic gonadotropin

A hormone present in both men and women, but is produced in large quantities by the placenta in a pregnant woman. Some pregnancy tests analyse the level of this hormone in the body.

Magnetic resonance imaging

A type of imaging that uses a magnetic field and low-energy radio waves, instead of X-rays, to obtain images of organs.

Pelvic examination

An examination performed by your doctor or nurse that involves a speculum examination with a duck-bill instrument and an internal examination in which they may put two gloved fingers inside your vagina to check for lumps or tender regions.

Ultrasound

A scan that uses high-frequency soundwaves to produce images of the body’s internal structures.

Uterus

The hollow organ of the female reproductive system that is responsible for the development of the embryo and fetus during pregnancy. Also known as the womb.

Gestational trophoblastic disease

A group of tumours, not necessarily cancerous, that originate in the uterus, from the cells that normally develop into the placenta (the attachment of the embryo to the uterus).

1. Royal Women’s Hospital (Melbourne Vic). (2014) The Women’s health book: a complete guide to health & wellbeing for women of all ages. North Sydney N.S.W.: Random House Australia.

Types of treatment

In most cases, the main treatment for molar pregnancy is to remove the abnormal placental tissue.

Treatments that may be recommended include:

  • Evacuation of the uterine contents, and;
  • Hysterectomy (which may be an option for older women who do not want any more children).

In a molar pregnancy, there will often be continued symptoms, such as vaginal bleeding following a miscarriage. Evacuation of the uterine contents may be recommended to ensure that all of the abnormal placental tissue has passed out of the uterus. This can be done using surgery or medication depending on individual circumstances. 

Gestational trophoblastic disease often requires treatments such as chemotherapy.

Follow-up monitoring

If any of the abnormal placental cells remain in the uterus, they can develop into more serious forms of gestational trophoblastic disease. To monitor this, levels of hCG are regularly tested in the weeks and months after the end of the pregnancy, to ensure it returns to normal levels. This may be done by urine or blood tests.

To ensure hCG levels return to normal, you will generally be advised to avoid getting pregnant again for a certain period of time.

Women who have had one molar pregnancy have an increased risk (about one in 100) of having it occur again in a later pregnancy. [1]  To detect any problems early, you may be recommended to have:

  • An early ultrasound during any future pregnancies, and; 
  • A blood test to determine your levels of hCG six weeks after the end of any future pregnancy.

Counselling

When a pregnancy ends, it can be very distressing and recovering emotionally can take time. Speaking to your doctor or a mental health professional can help. Counselling services may be available at your hospital.

Female patient talking with psychologist.Counselling can assist with the emotional recovery from a molar pregnancy. 

Cells

The fundamental unit of life; the simplest living unit that can exist, grow, and reproduce independently. The human body is composed of trillions of cells of many kinds.

Chemotherapy

A medication-based treatment, usually used in the treatment of cancers. There are numerous, different types of chemotherapy drugs that can be prescribed by a specialist. These can commonly be used alongside other cancer treatments such as surgery and radiotherapy.

hCG

A hormone present in both men and women, but is produced in large quantities by the placenta in a pregnant woman. Some pregnancy tests analyse the level of this hormone in the body.

Hysterectomy

Hysterectomy is the surgical removal of a woman's uterus (womb). A total hysterectomy involves removal of the uterus and cervix.

Ultrasound

A scan that uses high-frequency soundwaves to produce images of the body’s internal structures.

Uterus

The hollow organ of the female reproductive system that is responsible for the development of the embryo and fetus during pregnancy. Also known as the womb.

Gestational trophoblastic disease

A group of tumours, not necessarily cancerous, that originate in the uterus, from the cells that normally develop into the placenta (the attachment of the embryo to the uterus).

Placental

Relating to the placenta, the organ that forms within the uterus of a pregnant woman to provide her unborn baby with nourishment from the blood supply.

1. Royal Women’s Hospital (Melbourne Vic). (2014) The Women’s health book: a complete guide to health & wellbeing for women of all ages. North Sydney N.S.W.: Random House Australia.

Prognosis

In most cases, treatment of a molar pregnancy by evacuation of the uterus is successful. Most women can have normal pregnancies in the future, although there is about a one in 100 chance of having another molar pregnancy. Having regular check-ups and following your doctor's instructions can help to detect any signs of more serious gestational trophoblastic disease.

Uterus

The hollow organ of the female reproductive system that is responsible for the development of the embryo and fetus during pregnancy. Also known as the womb.

Gestational trophoblastic disease

A group of tumours, not necessarily cancerous, that originate in the uterus, from the cells that normally develop into the placenta (the attachment of the embryo to the uterus).

1. Royal Women’s Hospital (Melbourne Vic). (2014) The Women’s health book: a complete guide to health & wellbeing for women of all ages. North Sydney N.S.W.: Random House Australia.

Prevention

While molar pregnancies cannot be prevented, seeking antenatal care once you know you are pregnant can help to detect signs early. 

Antenatal

Before the birth of a baby.

1. Royal Women’s Hospital (Melbourne Vic). (2014) The Women’s health book: a complete guide to health & wellbeing for women of all ages. North Sydney N.S.W.: Random House Australia.