If a miscarriage happens during the first trimester of pregnancy (the first three months), it is usually due to problems with the foetus (the unborn baby).
If a miscarriage happens during the second trimester of pregnancy (between weeks 14 and 26), it is usually the result of an underlying health condition in the mother.
First trimester miscarriages
Most first trimester miscarriages are caused by problems with the chromosomes of the foetus.
Chromosomes are blocks of DNA. They contain a detailed set of instructions that control a wide range of factors, from how the cells of the body develop to what colour eyes a baby will have.
For a pregnancy to be successful, a foetus needs 46 chromosomes: 23 are from the father's sperm and 23 are from the mother's egg.
Sometimes, something can go wrong at the point of conception and the foetus receives too many or not enough chromosomes. The reasons for this are often unclear, but it means that the foetus will not be able to develop normally, resulting in a miscarriage.
Problems with chromosomes often happen by chance. However, a number of known risk factors increase the chances of these problems occurring.
The most important risk factor for miscarriage is probably the age of the mother:
- Women under 25 have a 9% risk of having a miscarriage.
- Women between 25 and 29 have an 11% risk of having a miscarriage.
- Women between 30 and 34 have a 15% risk of having a miscarriage.
- Women between 35 and 39 have a 25% risk of having a miscarriage.
- Women between 40 and 44 have a 51% risk of having a miscarriage.
- Women over 45 years have a 75% risk of having a miscarriage.
Other risk factors
Other risk factors for having a miscarriage include:
smoking during pregnancy,
- drug misuse during pregnancy (particularly cocaine),
- drinking more than 200mg of caffeine a day (equivalent of two mugs of tea or instant coffee, one mug of filter coffee or five cans of cola), and
- drinking more than two units of alcohol a week (one unit is half a pint of bitter or ordinary strength lager, a small glass of wine or a 25ml measure of spirits).
Second trimester miscarriages
Chronic health conditions
There are a number of chronic (long-lasting) health conditions that can increase the risk of having a miscarriage. These are:
There are a number of infections that may increase the risk of having a miscarriage. These include:
- rubella (German measles),
- cytomegalovirus (CMV), and
- toxoplasmosis (a bacterial infection).
Miscarriage can also be caused by a bacterial infection of the vagina. This type of infection is known as bacterial vaginosis (BV).
Antibodies are proteins that are produced by the immune system to fight infection.
Approximately 15% of women with a history of recurrent miscarriages (three or more miscarriages in a row) have a higher than usual level of an antibody called antiphospholipid (aPL) in the blood. The aPL antibodies are known to cause blood clots. These blood clots can block the supply of blood to the foetus, which can cause a miscarriage.
Having a high number of aPL antibodies in your blood is known as Hughes syndrome. See Useful links for more information about Hughes syndrome.
Problems and abnormalities with the womb can also lead to second trimester miscarriages. Possible problems with the structure of the womb include:
fibroids (non-cancerous growths in the womb), and
- scarring on the surface of the womb.
In some cases, the muscles of the cervix (the opening of the womb) are weaker than usual. This is known as a weakened cervix.
The muscle weakness can cause the cervix to open too early during pregnancy, leading to a miscarriage.
Prolactin is a hormone that is produced during pregnancy. It helps prepare the breasts for breastfeeding. Sometimes, women have a higher level of prolactin in their body than usual. This is known as hyperprolactinaemia.
Some limited evidence suggests that hyperprolactinaemia may be linked to an increased risk of miscarriage.
Polycystic ovary syndrome
Polycystic ovary syndrome (POS) is a condition where the ovaries are larger than normal. It can lead to hormonal imbalances inside the womb.
POS is known to be a leading cause of infertility. There is some evidence to suggest that it may also be linked to an increased risk of miscarriage in women who are still fertile.
However, the exact role that POS plays in miscarriages is unclear.
Misconceptions about miscarriage
There are a number of widely held assumptions about the possible causes of miscarriages. For example, maternal stress is often believed to be a cause. However, there is no evidence to support such claims.
An increased risk of miscarriage is also not linked to:
- a mother's emotional state during pregnancy, such as being stressed or depressed,
- having a shock or fright during pregnancy,
exercise during pregnancy (but discuss what type of exercise is suitable for you during pregnancy with your GP or midwife),
- lifting or straining during pregnancy,
- working during pregnancy, and
- having sex during pregnancy.
Chromosome: Chromosomes are the parts of a cell that carry genes. A human cell usually has 23 pairs of chromosomes.
Uterus: The uterus, or womb, is a hollow, pear-shaped organ in a woman where a baby grows during pregnancy.
Constipation: Constipation is when you pass stools less often than usual or when you have difficulty going to the toilet because your stools are hard and small.
Genetic: Genetic refers to genes, the characteristics inherited from a family member.