Causes of Miscarriage

Antiphospholipid Syndrome (APS)

This blood clotting problem is the most important treatable cause of recurrent miscarriage. It happens when your immune system makes abnormal antibodies that attack fats called phospholipids in your blood. This makes the blood more ‘sticky’ and likely to clot, which is why APS is sometimes called ‘sticky blood syndrome’. It is also known as ‘Hughes syndrome’ after the expert who named it.

Other Blood Clotting Problems

Some inherited blood clotting disorders can cause recurrent miscarriage, particularly after 14 weeks. These include factor V Leiden, factor II (prothrombin), gene mutation and protein S deficiency.

Abnormal Chromosomes

About half of all miscarriages happen because the baby’s chromosomes are abnormal. This is not usually an inherited problem: it happens when the egg and sperm meet or soon after the egg is fertilised. The older you are the more likely this is to happen. Much less commonly (in less than five in one hundred couples with recurrent miscarriage), one partner carries a chromosomal defect called a ‘balanced translocation’. This doesn’t cause a problem for the parent, but it can be passed on to the baby as an ‘unbalanced translocation’. This means that some genetic information is duplicated and some is missing.

Cervical weakness (also known as ‘Incompetent Cervix’)

Your cervix is a kind of ‘gateway’ between the uterus and vagina, which normally dilates (widens) during labour to allow the baby to be born. Some women – probably less than one in a hundred – have a weakness in the cervix that allows it to dilate too early. This is a known cause of late (second trimester) miscarriage.

There are treatments and tests available.  It is recommended that you and your partner be seen by expert health professionals, ideally at a special recurrent miscarriage clinic. Your doctor should know whether there is one in your area and organise a referral.

Causes of Miscarriage

The main causes of miscarriage are thought to be:

  • Genetic

    This is when the baby doesn’t develop normally right from the start and cannot survive.  This is the cause of more than half of all early miscarriages.

  • Hormonal

    Women with hormonal irregularities may find it harder to get pregnant; and when they do, are more likely to miscarry.

  • Blood-clotting problems

    Problems in the blood vessels that 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 very high fevers 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 can cause a miscarriage.
    • If the 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.


If this is your first or second miscarriage – or if you have had both miscarriages and healthy pregnancies – you probably won’t be offered tests or treatment.

This can be frustrating and upsetting. 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 not due to an underlying cause but rather genetics.

If you’ve had three miscarriages or more in a row (the definition of recurrent miscarriage), you should be offered tests.  That’s because a cause is more likely to be found at this stage.  You may also be offered tests after a second trimester loss or after two miscarriages if it has taken you a long time to conceive.

It’s important to know that having tests does not necessarily mean that a cause or causes will be found.

Only a small number of women who have investigations after recurrent miscarriage will be given a definite cause for 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.

Talk to your consultant or GP who should be able to give you the option of a range of blood tests to start with.