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24th May 2019

Molar Pregnancy

Consultant Gynaecologist Narendra Pisal defines molar pregnancy, explaining its symptoms, treatment and risk factors. 

Molar pregnancy is a rare complication of pregnancy where the fetus and placenta do not develop normally and leads to a non-viable pregnancy. It occurs in around 1 in 600 to 1 in 1000 pregnancies. It is also known as Hydatidiform Mole or Gestational Trophoblastic Disease. Molar pregnancy can be partial or complete. Partial molar pregnancy is where there is a fetus with abnormal placentation where as in complete molar pregnancy, there is no fetus, just abnormal placental tissue.

Signs and symptoms 

Many molar pregnancies are asymptomatic and are diagnosed during a routine early pregnancy ultrasound scan. The placenta is seen to be abnormal with or without a non-viable fetus on ultrasound scan. Sometimes, diagnosis is only made when tissue is sent for analysis after a miscarriage. Common symptoms include morning sickness, abdominal pain and vaginal bleeding. The uterus is also more enlarged than expected gestational age. Excessive morning sickness is caused by higher levels of pregnancy hormones (beta hCG). Abdominal pain is caused by rapidly expanding uterus and sometimes a cyst of pregnancy (luteal cyst of ovary). Vaginal bleeding can be a sign of miscarriage.

Causes and risk factors

Molar pregnancy is caused by faulty fertilisation process. Complete molar pregnancy occurs when an empty egg (no chromosomes) is fertilised by two sperms. Thus two paternal sets of chromosomes are seen in a complete mole. Partial mole occurs when a normal egg follicle is fertilised by two sperms thus leading to three sets of chromosomes (triploidy). The risk factors include extremes of ages (teenage mothers or women over 45 years of age), Asian ethnicity and previous history of molar pregnancy.


Molar pregnancy is treated by surgical evacuation of pregnancy under ultrasound guidance. It is important to register all molar pregnancies with specialist centres in London, Sheffield or Dundee. In London, the centre is located in Charing Cross Hospital.

Monitoring of beta hCG (pregnancy hormone) levels for six months after surgical evacuation is important to ensure that it does not develop into persistent trophoblastic disease (also known as GTN or Gestational Trophoblastic Neoplasia). The specialist centres would usually do this by inviting women to send urine samples by post.

Sometimes a repeat ultrasound scan and repeat surgical evacuation may be needed if there is persistent pregnancy tissue. Very rarely, this tissue can have malignant potential or indeed become malignant and may need to be treated by chemotherapy.

Sex, contraception and pregnancy after molar pregnancy 

It is important to avoid further conception for at least six months. Best contraception often is barrier contraception (condoms) as hormonal contraception and IUDs are relatively contraindicated. In future pregnancies, an early ultrasound scan and beta hCG levels are necessary as the risk is slightly higher (around 1 in 100). Beta hCG monitoring needs to continue for six months after delivery and is organised by the specialist centres.

Connection to abnormal (or cancerous) cells 

In a small proportion of molar pregnancies, the pregnancy tissue can become persistent and sometimes cancerous. This type of cancer is called as Choriocarcinoma. This cancer is however very sensitive to chemotherapy and the prognosis is often very good. Monitoring of beta hCG levels is therefore critical for six months after a molar pregnancy.

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