Causes of miscarriage

There are many reasons why a miscarriage may happen, although the cause isoften 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). Aboutthree in everyfour 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.

Chromosome problems

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.

Placental problems

The placenta is the organ linking the mother's blood supply to her baby's. If there'sa 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 toincreaseyour riskof problems happening.

Theage 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 thanhalf of all pregnancies will end in miscarriage

Other risk factors include:

  • Obesity
  • 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 weekone unit is half a pint of bitter or ordinary strength lager, or a 25ml measure of spirits, anda small 125ml glass of wine is 1.5 units

Second trimester miscarriages

Long-term health conditions

Severallong-term (chronic) health conditions can increase your risk of having a miscarriage in the second trimester. These are:

  • diabetes (if it's poorly controlled)
  • severe high blood pressure
  • lupus
  • kidney disease
  • an overactive thyroid gland
  • an underactive thyroid gland


The following infections may also increase your risk:

  • rubella (german measles)
  • cytomegalovirus
  • bacterial vaginosis
  • HIV
  • chlamydia
  • gonorrhoea
  • syphilis
  • malaria

Food poisoning

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 caughtby eating raw or undercooked infected meat, particularly lamb, pork or venison
  • salmonella most often caused by eating raw or partly cooked eggs

Womb structure

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

Weakened cervix

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

Recurrent miscarriages

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) andmore than60%of these women go on to have a successful pregnancy.

Content supplied by the NHS Website

Medically Reviewed by a doctor on 16 Mai 2016