What are the causes of a miscarriage?

In the majority of cases, miscarriages occur as a result of problems relating to the chromosomes in the foetus; this is most common during the first trimester of the pregnancy (the first 12 weeks). Chromosome disorders are usually caused by having additional chromosomes or not enough chromosomes; it is not known why these abnormalities occur but they can cause problems with development, learning and growth. The most well-known chromosome disorder is Down’s syndrome, which is caused by having an extra chromosome 21.

There are a number of risk factors which may increase the chances of having a miscarriage during the first trimester; these include:

• Age (women aged over 35 are much more likely to have a miscarriage than younger women; women under the age of 25 have a 9% chance of miscarrying, while those aged between 35 and 39 have a 25% chance of miscarriage)
• Obesity
• Smoking during pregnancy
• Drinking alcohol during pregnancy
• Taking drugs during pregnancy
• Drinking large amounts of caffeine during pregnancy (drinking more than the equivalent of 2 cups of tea or coffee per day)

Most second trimester miscarriages (weeks 14-26) are caused by health conditions which affect the mother. There are a number of long-term health conditions, which may increase the risk of having a miscarriage; these include:

• High blood pressure (also known as hypertension)
• Diabetes (this should not affect the baby if it is properly controlled and monitored)
• Kidney disease
• Lupus (this is a condition which causes the immune system to attack healthy body tissue)
• Problems with the thyroid gland (either overactive or underactive thyroid gland)

The chance of miscarriage may also be increased by short term illnesses, which are caused by infection; examples of these include:

• German measles (also known as Rubella)
• Toxoplasmosis
• CMV (cytomegalovirus)

Other possible causes of second trimester miscarriages include:

• Antibodies: some people suffer from Hughes syndrome, which causes them to have a high level of aPL antibodies (antiphospholipid). Having a high number of aPL antibodies can cause the blood to clot, preventing blood from getting to the foetus.
• Problems with the structure of the womb or cervix; examples of these problems include fibroids, scarring on the tissue of the womb and weaker muscles in the cervix (this causes the cervix to open prematurely during pregnancy).
• High levels of prolactin (also known as hyperprolactinaemia): prolactin is produced by the body during pregnancy to prepare for breastfeeding. If the levels are too high this may increase the risks of a miscarriage although research to confirm this is very limited.
• PCOS (polycystic ovarian syndrome); Instances where the ovaries are of a greater size than normal, and as well as being linked to miscarriage is also a foremost cause of infertility (see the relevant infertility guide for more information)