There are many reasons why a miscarriage may happen, although the cause is often not identified.
If a miscarriage happens during the first trimester of pregnancy (the first three months), it's usually caused by problems with the unborn baby (foetus). About three in every four miscarriages happen during this period.
If a miscarriage happens during the second trimester of pregnancy (between weeks 14 and 26), it may be the result of an underlying health condition in the mother.
These late miscarriages may be caused by an infection around the baby, which leads to the bag of waters breaking before any pain or bleeding. In rare cases, they can be caused by the neck of the womb opening too soon.
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.
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 the foetus won't be able to develop normally, resulting in a miscarriage.
It's estimated up to two-thirds of early miscarriages are associated with chromosome abnormalities. This is very unlikely to recur and doesn't mean there's any problem with the mother or father's chromosomes.
The placenta is the organ linking the mother's blood supply to her baby's. If there's a problem with the development of the placenta, it can also lead to a miscarriage.
Things that increase your risk
An early miscarriage may happen by chance. But there are several things known to increase your risk of problems happening.
The age of the mother has an influence:
- 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 include:
- smoking during pregnancy
- drug misuse during pregnancy
- 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; caffeine is also found in some fizzy drinks, energy drinks and chocolate bars
- drinking more than two units of alcohol a week – one unit is half a pint of bitter or ordinary strength lager, or a 25ml measure of spirits, and a small 125ml glass of wine is 1.5 units
Second trimester miscarriages
Long-term health conditions
Several long-term (chronic) health conditions can increase your risk of having a miscarriage in the second trimester. These are:
The following infections may also increase your risk:
Food poisoning, caused by eating contaminated food, can also increase the risk of miscarriage. For example:
- listeriosis – most commonly found in unpasteurised dairy products, such as blue cheese
- toxoplasmosis – which can be caught by eating raw or undercooked infected meat, particularly lamb, pork or venison
- salmonella – most often caused by eating raw or partly cooked eggs
Read more about foods to avoid in pregnancy.
Medicines that increase your risk include:
- 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 (NSAIDs) – such as ibuprofen; these are used for pain and inflammation
To be sure a medicine is safe in pregnancy, always check with your doctor, midwife or pharmacist before taking it.
Read more about medicines during pregnancy.
Problems and abnormalities with your womb can also lead to second trimester miscarriages. Possible problems include:
- non-cancerous growths in the womb called fibroids
- an abnormally shaped 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 caused by a previous injury to this area, usually after a surgical procedure. The muscle weakness can cause the cervix to open too early during pregnancy, leading to a miscarriage.
Polycystic ovary syndrome
Polycystic ovary syndrome (PCOS) is a condition where the ovaries are larger than normal. It's caused by hormonal changes in the ovaries.
PCOS is known to be a leading cause of infertility as it can lower the production of eggs. There's some evidence to suggest it may also be linked to an increased risk of miscarriages in fertile women.
However, the exact role polycystic ovary syndrome plays in miscarriages is unclear. No treatment has been proven to make a difference and the majority of women with PCOS have successful pregnancies with no increased risk of miscarriage.
Misconceptions about miscarriage
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 with your GP or midwife what type and amount of exercise is suitable for you during pregnancy
- lifting or straining during pregnancy
- working during pregnancy – or work that involves sitting or standing for long periods
- having sex during pregnancy
- travelling by air
- eating spicy food
Many women who have a miscarriage worry they'll have another if they get pregnant again. But most miscarriages are a one-off event.
About 1 in 100 women experience recurrent miscarriages (three or more in a row) and more than 60% of these women go on to have a successful pregnancy.