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What happens if a foetus is not formed after fertilisation?

Though rare, it is possible for a foetus not to be formed after fertilisation. In a normal pregnancy, when a sperm fertilises an egg, both cells contribute equally in donating their chromosomes to the developing foetus. Sometimes, however, this process is disrupted, leading to the growth of abnormal tissue in the uterus, which can result in a molar pregnancy or a mole.

A molar pregnancy (or a mole) is when a foetus is not formed normally after fertilisation. There are two possible scenarios, one of which results in a complete mole, the other, a partial mole. A complete mole happens when the sperm fertilises an empty egg (i.e. the egg contains no genetic material), and this leads to the development of only placental tissue. There is no foetus in a complete mole and the pregnancy cannot proceed. A partial mole is usually the result of too many chromosomes present at fertilisation. This leads to abnormal tissue as well as a foetus. Unfortunately, it means that the foetus usually has severe defects, which are often fatal.

Detection of molar pregnancies

From the start, these pregnancies are not normal, and most women will experience warning signs early on. The majority of women with a molar pregnancy will have bleeding in the first trimester. Bleeding during pregnancy is always concerning. One of the first steps your doctor will probably take is to perform an ultrasound to try to identify the cause. This is one way that a molar pregnancy can be detected. Alternatively, if hCG levels in the blood are being monitored, your doctor or midwife may notice that they are reaching abnormally high levels. In fact, the levels of hCG can be so high that they result in severe morning sickness (hyperemesis gravidarum). Other warning signs include a uterus that measures larger than it should for the gestational age, and abnormal cysts on the ovaries

Treating a molar pregnancy

Treatment is usually removal of the mole using dilation and curettage (D&C). If you know you do not want to have any more children, a hysterectomy is an alternative option. Leaving the abnormal tissue in a woman’s uterus is not recommended as the bleeding can worsen and cause substantial health risks. In order to ensure that all the tissue has been removed, a doctor will regularly measure hCG levels in the blood to make sure they are falling. They should become undetectable, however this can take weeks, or even months from initial diagnosis and treatment of the molar pregnancy. It is very important to not get pregnant whilst hCG levels are still elevated. You should speak to your gynaecologist about how best to avoid this from happening.

Why is it so important that the hCG levels go back to normal before you attempt to conceive again? If the molar pregnancy is not fully removed, the tissue that remains can persist and form a disease called persistent trophoblastic disease (PTD). This is less common in partial moles (1 in 200; 0.5%) than in complete moles (1 in 7; 15%). PTD can be very serious because it can grow like a cancer and spread to other organs in the body. Treatment involves chemotherapy to kill the abnormal cells. Success rates are high and, with the correct treatment, almost all women make a full recovery.

How common are molar pregnancies?

Whilst all of this sounds very scary, it is important to remember that molar pregnancies are rare; global incidence rates vary, but are estimated to be in the region of 1 out of every 600 pregnancies. 

Having a previous molar pregnancy does increase the risk of having another, to 1 in 80 and if you have two or more, your risk is 1 in 5. This is why women with a history of molar pregnancies should ensure that they establish prenatal care very early in subsequent pregnancies so that early investigative procedures can be performed.


  • “Hydatidiform Mole.” Background, Pathophysiology, Epidemiology, Medscape, 2 Feb. 2019, emedicine.medscape.com/article/254657-overview#a6.
  • “Molar Pregnancy.” NHS Choices, NHS, www.nhs.uk/conditions/molar-pregnancy/
  • Soper, J T, et al. “ACOG Practice Bulletin #53: Diagnosis and Treatment of Gestational Trophoblastic Disease.” Obstetrics & Gynecology, vol. 103, no. 6, 2004, pp. 1365–1377., doi:10.1097/00006250-200406000-00051.

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