If a miscarriage happens during the first trimester of pregnancy (the first three months), it is usually due to problems with the unborn baby (foetus).
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 to have 46 chromosomes in total:
- 23 are from the father’s sperm
- 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.
It is estimated that up to two thirds of early miscarriages are associated with chromosome abnormalities.
The placenta is the organ that links the mother’s blood supply to her baby’s. If there is a problem with the development of the placenta it can also lead to a miscarriage.
An early miscarriage may happen by chance. However, there are several known risk factors which increase the chances of problems occurring.
One of the most important risk factor for miscarriage is the age of the mother:
- In women under 30, 1 in 10 pregnancies will end in miscarriage.
- In women aged 35-39, up to 2 in 10 pregnancies will end in miscarriage.
- In women over 45, more than half of all pregnancies will end in 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: one mug of tea contains around 75mg of caffeine, and one mug of instant coffee contains around 100mg of caffeine
- 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
Long-term health conditions
There are several long-term (chronic) health conditions that can increase the risk of having a miscarriage. These are:
There are some infections that may increase the risk of having a miscarriage. These include:
Some medicines can also increase the risk of miscarriage:
- misoprostol (used for conditions such as rheumatoid arthritis)
- retinoids (used for eczema and acne)
- methotrexate (used for conditions such as rheumatoid arthritis)
- non-steroidal anti-inflammatory drugs (used for pain and inflammation)
Read more about medicines during pregnancy. To be sure that a medicine is safe in pregnancy, always check with your doctor, midwife or pharmacist before taking it.
Antibodies are proteins that are produced by the immune system (the body’s natural defence system) to fight infection.
Some women who have had three or more miscarriages in a row (recurrent miscarriages) have a higher than usual level of an antibody called antiphospholipid (aPL) in their 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. Read more 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:
- non-cancerous growths in the womb called fibroids
- scarring on the surface of the womb
In some cases, the muscles of the cervix (neck of the womb) are weaker than usual. This is known as a weakened cervix or cervical incompetence. A weakened cervix may be due to a previous injury to this area, or may have been something you were born with.
The muscle weakness can cause the cervix to open too early during pregnancy, leading to a miscarriage.
Prolactin is a hormone which is produced during pregnancy. Prolactin helps to 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 (PCOS) is a condition where the ovaries are larger than normal. It can lead to hormonal imbalances inside the womb.
Polycystic ovary syndrome 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 polycystic ovary syndrome plays in miscarriages is unclear.
Read more about polycystic ovary syndrome.
Misconceptions about miscarriage
There are a number of widely held assumptions about the possible causes of miscarriages. However, there is no evidence to support such claims.
An increased risk of miscarriage is 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
- having sex during pregnancy