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Causes, tests and treatment

Why did I miscarry?  (see information for ectopic pregnancy and molar pregnancy below)

Even though about one in four pregnancies ends in miscarriage, there’s a lot that we still don’t know about why it happens.  That means that most women never find out the cause of their loss, even if they have investigations.

It can be very difficult to cope with not having an obvious reason for your miscarriage.  If you know why it happened, that can help make sense of it all, and perhaps help you plan for another pregnancy. If you don’t know, you may begin to wonder whether it was somehow your or your partner’s fault.

It’s important to know that that your miscarriage (or ectopic or molar pregnancy) is very unlikely to have happened because of anything you did or didn’t do.

The main causes of miscarriage are thought to be:

  • Genetic: In about half of all early miscarriages, the baby does not develop normally right from the start and cannot survive.
  • Hormonal: Women with very irregular periods may find it harder to conceive and when they do, are more likely to miscarry.
  • Immunological/ blood-clotting: Problems in the blood vessels which supply the placenta can lead to miscarriage, especially if the blood clots more than it should.
  • Infection: Minor infections like coughs and colds are not harmful, but a very high temperature and some illnesses or infections, such as German measles, may cause miscarriage.
  • Anatomical: There are three main anatomical causes of miscarriage:
  • If the cervix (the bottom of the uterus) is weak, it may start to open as the uterus becomes heavier in later pregnancy and this may lead to miscarriage.
  • If your uterus has an irregular shape, there may not be enough room for the baby to grow.
  • Large fibroids (harmless growths in the uterus) may cause miscarriage in later pregnancy.

For more information about causes, you may like to read our leaflet Why me?

 What about investigations or tests?

Most hospitals offer to do tests or investigations only if a woman has had three or more miscarriages in a row (the definition of recurrent miscarriage). About 1 in 100 couples will experience recurrent miscarriage.  If you have had fertility problems or second trimester loss, you may be offered investigations after two miscarriages.

This can be frustrating and upsetting if you have had one or two miscarriages, or if you had, say, two miscarriages and then a baby and then another miscarriage. You might feel that no-one is taking your losses seriously.

The reason for this policy, though, is because most women who have one or two miscarriages will go on to have a successful pregnancy next time. This suggests that their miscarriages were due to chance rather than to an underlying cause.

It’s also important to know that having tests does not necessarily mean that a cause or causes will be found. About half of the couples who have investigations don’t come out with any clear finding of why they miscarried. Again, this can be frustrating, but it is also positive news because it means that there is a good chance of the next pregnancy being successful, without any treatment at all.

If a problem is identified, there may still be a good chance of having a successful pregnancy. This will depend on what is found and whether there is any treatment to reduce the risk next time.

There is more information about investigations and tests in our leaflet ­Recurrent miscarriage .


There is no single treatment that can prevent every kind of miscarriage, just as there is no single cause of miscarriage. But some treatments have been shown to improve the chances of a healthy pregnancy in particular cases. And there are also some general guidelines about reducing the risk of miscarriage.

Some miscarriages are caused by chance (e.g. a chromosome abnormality in the baby) and cannot be predicted or prevented. Fortunately, they are not very likely to happen again.

Some causes of miscarriage cannot be treated because as there is no way of changing the basic problem (e.g. a problem with the parents chromosomes), but parents may be advised to consider other options (like using donor eggs or sperm).

But in most cases, couples have no idea why they have miscarried, so there is no treatment to recommend.

This can feel frustrating, especially since every now and again, there is a story in the press about a new treatment for miscarriage. There’s also a great deal of information on the Internet about miscarriage causes and treatments. The problem is that it can be difficult to judge how accurate this information is, as many investigations and treatments which are reported have not been properly studied or assessed. So while some treatments might be useful in some cases, they may not be suitable for you. Others may not be effective, or they may even be harmful.

If you have any questions or doubts about what you have read or heard, you might like to:

 Ectopic pregnancy

The word “ectopic” means “out of place”.  An ectopic pregnancy is one where the baby starts to develop outside the uterus.  It is usually in one of the Fallopian tubes – the place where sperm and egg meet and where the egg is fertilised – but it can be at the very top of the uterus, outside it, or somewhere else in what is called the abdominal cavity.

Why and how does that happen?

As with miscarriage, there are some known reasons for ectopic pregnancy and a lot that is still being researched.  In many cases, there is no clear cause.  But known causes are:

  • Narrow or blocked Fallopian tube/s. Normally, the fertilised egg is moved along by tiny hairs (fimbriae) inside the tube till it reaches the uterus, where it implants (burrows into the lining of the uterus).  If anything prevents or slows down that journey – such as narrowing or blockages in the tube – the egg will start to implant wherever it happens to be – probably in the tube. Narrowing or blockage may be caused by:
  • Pelvic infection
  • Previous abdominal surgery, e.g. for appendicitis
  • Endometriosis
  • A problem with the fimbriae, the tiny hairs inside the tube.  If these don’t work properly, the fertilised egg may implant inside the tube or it may never get that far, and begin growing elsewhere.
  • Other.  Ectopic pregnancy is known to be much more common in women who smoke, in women under 18 and those over 40.

There is more information in our leaflet Ectopic pregnancy.

 Molar pregnancy (also called Hydatidiform mole pregnancy)

Hydatidiform mole is a medical term which means a fluid-filled mass of cells (mole = a mass of cells; hydatid = containing fluid-filled sacs or cysts).

In a molar pregnancy, 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.

About one in 600 pregnancies is a molar pregnancy so it is quite rare, especially compared with miscarriage, which affects around one in four pregnancies.

Why and how does molar pregnancy happen?

It starts with an abnormal fertilised egg, which either has too much or not enough genetic information.

In a normal conception, a single sperm with 23 chromosomes fertilises an egg with 23 chromosomes, making 46 in all.  Each of our cells contains 23 pairs of chromosomes, where one of each pair is from the mother and the other from the father.

Sometimes this process goes wrong, and one possible outcome is a molar pregnancy. The hydatidiform mole may be either partial or complete, depending on the genetic make-up of the fertilised egg.

  • In a partial mole, two sperm fertilise the egg instead of one, creating 69 instead of 46 chromosomes. With too much genetic material, the pregnancy develops abnormally, with the placenta outgrowing the baby. There may be evidence of a fetus but it will be abnormal and cannot survive.
  • In a complete mole, one (or even two) sperm fertilises an egg cell that has no genetic material inside. There is not enough genetic material. Usually the fertilised egg dies at that point but in rare cases it goes on to implant in the uterus. When it does, the embryo doesn’t develop, just the cells that will become the placenta and they fill the uterus with molar tissue.

If you are diagnosed as having a molar pregnancy, you will need surgery to remove the remains of the pregnancy and the molar tissue, and then follow-up in a specialist centre.  You will be advised not to try to get pregnant again until follow-up is complete.