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Molar pregnancy - symptoms, diagnosis, treatment

A molar pregnancy occurs when the tissue surrounding a fertilised egg develops abnormally. A range of symptoms may appear by the fourth month of pregnancy but often molar pregnancy is discovered when a miscarriage occurs.

Treatment usually includes a dilation and curettage (D&C) following the miscarriage to remove molar tissue in the uterus that, if not removed, has a small risk of developing into cancer.

In New Zealand, it is estimated that 1 in every 1000 pregnancies will be a molar pregnancy.
A molar pregnancy is also known as a hydatidiform mole. The tissue surrounding the fertilised egg, which would normally develop into the placenta, instead forms a grapelike mass inside the uterus. There are two different types of molar pregnancy:

Partial molar pregnancy
In addition to the abnormal tissue, some normal pregnancy tissue develops, eg: a foetus, amniotic tissue, or umbilical cord. If a foetus develops, it will nearly always die early in the pregnancy as a result of the condition. Only very rarely in a partial molar pregnancy does a foetus survive to full term.

Complete molar pregnancy
In this form of molar pregnancy, no normal pregnancy tissue develops at all.  

Causes and risk factors

The cause of molar pregnancy is not fully understood but is thought to be related to problems with the genetic information of an egg or sperm.
Having a previous molar pregnancy increases the chance of having another molar pregnancy from 1 in a thousand to between 1 in 100 and 1 in 50. Maternal age is also a risk factor – a molar pregnancy is more likely in a woman aged younger than 20 years or older than 35 years.

The condition is usually benign (non-cancerous). However, molar pregnancies can become cancerous if not removed. The chances of a molar pregnancy becoming cancerous are higher with a complete molar pregnancy than with a partial molar pregnancy.

Signs and symptoms

Women with a molar pregnancy will have a positive pregnancy test and the same early symptoms of a normal pregnancy. For the first three to four months there may be no sign that anything is amiss with the pregnancy.

However, signs and symptoms of a molar pregnancy may then appear and can include:

  • Faster than usual growth of the uterus
  • Vaginal bleeding or a dark discharge from the vagina in early pregnancy
  • Nausea and vomiting, which may be severe
  • Passage of grapelike tissue from the vagina
  • Absence of foetal movement or heartbeat
  • High blood pressure
  • Hyperthyroidism – an overproduction of thyroid hormones, which can lead to weight loss and increased appetite.


In most cases, molar pregnancy is discovered when a miscarriage occurs. If a molar pregnancy is suspected before a miscarriage occurs, it can usually be detected by ultrasound.

Blood and urine tests may be used to detect abnormally high levels of human chorionic gonadotropin (hCG). This is a hormone normally produced during pregnancy but present at much higher levels with a molar pregnancy. An ultrasound scan may also be done.


In most cases the molar pregnancy will miscarry spontaneously by the fourth month of pregnancy. A dilatation and curettage (D&C) is usually performed a few days after miscarriage to be certain that no molar tissue is left in the uterus. D&C is a minor surgical procedure that is carried out under a general anaesthetic.

If the molar pregnancy does not spontaneously miscarry, the woman may be given a medication to trigger the uterus to release the pregnancy. Again, this is usually followed by a D&C.

If the molar pregnancy does not miscarry despite medication being given, a procedure called vacuum aspiration may be performed. This procedure is similar to a D&C. During the procedure, a general anaesthetic is given, the cervix is opened and the molar pregnancy is removed using a gentle suction instrument. The uterus is then checked to make sure all of the molar tissue has been removed.

For women who are older and do not want any more children, a hysterectomy may be considered, as this eliminates the chance of cancer developing as a result of the molar pregnancy.

Follow up

Because of the risk of abnormal molar cells left in the womb continuing to grow (persistent trophoblastic disease) or developing into cancer (choriocarcinoma), monitoring for between 2 and 12 months is usually required. Regular blood tests are performed to monitor the fall of hCG levels. Elevated levels of hCG after the pregnancy has ended can signal that not all of the molar tissue has been removed or that cancer is developing as a result of the molar pregnancy.

While the condition is being monitored it is important to avoid pregnancy. This may be for a period of up to one year, although recommendations for each woman will differ.


Auckland District Health Board (2015). Molar pregnancy (Pamphlet PDF). Auckland: ADHB Gynaecology Service Women’s Health.
Mayo Clinic (2014). Molar pregnancy (Web Page). Rochester, NY: Mayo Foundation for Medical Education and Research. [Accessed: 16/08/17]
NHS Choices (2017). Molar pregnancy (Web Page). Redditch: National Health Service (NHS)
England. [Accessed: 16/08/17]

Last reviewed – August 2017


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