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Molar pregnancy

Molar pregnancy (hydatidiform mole) can be a very distressing experience.  This section of our website aims to tell you something about what it is, why it happens and more.

One of the most difficult things about molar pregnancy is that most people have never heard of it, so it can be difficult to get the information and support you need.  We talk below about what molar pregnancy is and what we know about why it happens.  On the pages that follow, we’ll talk about

We hope this will help at what can be a very difficult time.

You can also find more detailed information in our leaflet on molar pregnancy.

What is a molar pregnancy?

A molar pregnancy, also called a hydatidiform mole, is one where an abnormal fertilised egg implants in the uterus (womb).  The cells that should become the placenta grow far too quickly and take over the space where the embryo would normally develop.

The term ‘hydatidiform mole’ means a fluid-filled mass of cells.  The word ‘mole’ means a mass of cells; and ‘hydatid’ means containing fluid-filled sacs or cysts.

Those cells are called trophoblasts.  That’s why molar pregnancy is sometimes called trophoblastic disease.

About one in 600 pregnancies is a molar pregnancy, so it’s quite rare.

Types of molar pregnancy

Molar pregnancies might be partial or complete.

In a partial mole, two sperm fertilise the egg instead of one.  There is too much genetic material for the baby to be able to develop.

In a complete mole, one sperm (or even two)  fertilises an egg cell that has no genetic material inside.  There are not enough of the right chromosomes for the baby to be able to develop.

In a very small number of cases, molar cells burrow more deeply into the uterus than they should.  These cells can become cancerous and spread into other parts of the body.  This is called invasive mole.  If invasive mole is not treated, it can develop into choriocarcinoma, which is a form of cancer.  Fortunately is it a cancer with a cure rate of almost 100%.

Can a molar pregnancy survive?

No.  Molar pregnancies all have the wrong number or balance of chromosomes and they can’t survive.

What causes molar pregnancy?

Molar pregnancy is a chance event.  Doctors understand how it happens but there are no obvious underlying causes or risk factor, except a previous molar pregnancy.  It just happens.

You can read about symptoms and diagnosis of molar pregnancy here.