Molar pregnancy

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 ultrasound scans are done in the first trimester of a pregnancy.

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

Causes and risk factors

A molar pregnancy is also known as a hydatidiform mole. It occurs when the tissue surrounding the fertilised egg, which would normally develop into the placenta, instead forms an abnormal 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.

The cause of molar pregnancy is related to problems with the sharing of genetic information of an egg or sperm.

In a normal pregnancy, an egg (from the mother) and a sperm (from the father) fuse together (fertilisation) and share their genetic material. In a complete molar pregnancy, a sperm fertilises an egg that contains no genetic material. The fertilised egg usually dies at that point but, rarely, it can implant in the womb resulting in an abnormal pregnancy.

A partial molar pregnancy occurs when two sperm fertilise an egg. In this case, there is too much genetic material and the pregnancy develops abnormally.

In New Zealand, it is estimated that one in every 1,200 pregnancies will be a molar pregnancy. Having a previous molar pregnancy increases the chance of having another molar pregnancy to between one in 100 and one 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.

Symptoms and diagnosis

In past years, most cases of molar pregnancy were discovered when a miscarriage occurred. However, with the development of high-resolution ultrasonography, most molar pregnancies are now diagnosed in the first trimester of pregnancy before signs and symptoms are noticed.

Women with a molar pregnancy will have a positive pregnancy test and the same early symptoms of a normal pregnancy. In the absence of medical intervention or diagnosis, the pregnancy might seem normal for the first three to four months.

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
• Anaemia due to vaginal bleeding
• Passage of tissue from the vagina
• Absence of foetal movement or heartbeat
• Pre-eclampsia – a complication of pregnancy that results in high blood pressure and swelling in the feet, ankles, and legs after 20 weeks of pregnancy
• Hyperthyroidism – an overproduction of thyroid hormones, which can lead to rapid heartbeat, high blood pressure, intolerance to heat, weight loss, and increased appetite.

A molar pregnancy can usually be diagnosed by high resolution ultrasound scans, because of the distinctive appearance of molar tissue. A complete molar pregnancy may be easier to detect by ultrasound than a partial molar pregnancy.

Blood and urine tests may also be used to detect abnormally high levels of human chorionic gonadotropin (hCG), abnormally high levels of which may indicate a molar pregnancy.

Other tests may be done to check for pre-eclampsia and hyperthyroidism.

Treatment

An undiagnosed molar pregnancy will in most cases miscarry spontaneously by the fourth month of pregnancy. A surgical procedure called dilation and curettage (D&C) is usually performed a few days after miscarriage to remove any remaining abnormal molar tissue from the uterus. D&C may also be done using vacuum aspiration (suction) to remove the molar tissue. D&C is a minor procedure that is carried out under a general anaesthetic.

When a molar pregnancy is diagnosed before a miscarriage, D&C will most likely be recommended due to the high risk of complications if the pregnancy is continued. Complications include bleeding, premature delivery, pre-eclampsia, and hyperthyroidism.

In some cases, a medication may be given to trigger the uterus to release the pregnancy followed by a D&C to remove remaining abnormal tissue.

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

Follow-up

Treatment is usually successful. However, because there is a small risk of abnormal molar cells left in the womb continuing to grow or developing into cancer, monitoring for between two 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.

References

Mayo Clinic (2017). Molar pregnancy (Web Page). Rochester, MN: Mayo Foundation for Medical Education and Research. https://www.mayoclinic.org/diseases-conditions/molar-pregnancy/symptoms-causes/syc-20375175 [Accessed: 31/08/20]
MedlinePlus (2018). Hydatidiform mole. Bethesda, MD: U.S. National Library of Medicine. https://medlineplus.gov/ency/article/000909.htm [Accessed: 31/08/20]
NHS (2017). Molar pregnancy (Web Page). Redditch: National Health Service (NHS)England. https://www.nhs.uk/conditions/molar-pregnancy/ [Accessed: 31/08/20]

Last reviewed: August 2020

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