Causes of recurrent miscarriage
For the most part, the causes of recurrent miscarriage are the same as the causes of a single miscarriage. However, there are certain risk factors that make miscarriage more likely to happen more than once.
We list some of these below, and you can find more detail in the RCOG’s very helpful Patient Information Leaflet on recurrent miscarriage.
Key risk factors
About half of all early miscarriages are caused by random, ‘one-off’ errors in the egg or the sperm, or in how the fertilised egg develops. It is not always clear what causes these faults, though they are more common in women in their late 30s or older, when egg quality declines.
(Women are born with all the egg cells they will ever have, so as they age, the egg cells do too and their quality also reduces. That in turn means it can take longer to become pregnant and if there is a pregnancy, there is a higher risk of the embryo having abnormal chromosomes.)
In a very few cases, one parent has an error in the way their chromosomes are arranged, called a ‘balanced translocation’. This doesn’t cause a problem for the parent, but it can be passed on to the baby as an ‘unbalanced chromosome’ and cause a miscarriage.
The risk of single and recurrent miscarriage increases with age. That is especially the case if you are over 35 and your partner (or the biological father) is over 40.
Your risk of miscarriage increases with the number of miscarriages that you have had in the past.
If you are of Black African or Black Caribbean background, you are more likely to have a miscarriage and, sadly, other pregnancy and maternity complications. Doctors do not yet know why this is and research into the possible reasons is a high priority.
Being very overweight (BMI over 25) or very underweight (BMI below 19) increases the risk of miscarriage. There is also some evidence that cigarette smoking and drinking more than the recommended maximum amount of alcohol or caffeine increase miscarriage risk.
I’ve found lots of information on the Internet, but different sites say different things. It’s really hard to know what to believe.
Other known causes of miscarriage
We note these in brief here, as there is more detail available in the RCOG Patient Information Leaflet mentioned above and in our leaflet Recurrent miscarriage.
Blood clotting problems
- Antiphospholipid syndrome (APS) – also sometimes called ‘sticky blood syndrome’. Your immune system makes abnormal antibodies that can cause recurrent early miscarriage. APS can also lead to later pregnancy loss and other complications, including the baby not growing enough, pre-eclampsia or stillbirth. It also increases the risk of developing blood clots. We have a leaflet on APS.
- Inherited blood clotting problems, known as thrombophilia. Some of these, such as factor V Leiden and protein S deficiency, are linked to a slightly increased risk of miscarriage.
Abnormally shaped uterus (womb) or other uterine problems
A small number of women (5-6 in 100) are born with an unusually shaped uterus. This is rather more common (13 in 100) in women who have recurrent miscarriage.
- A septate or bicornuate uterus, both divided down the centre to some extent, may increase your chance of miscarriage. (There’s a helpful diagram in the RCOG leaflet.)
- Fibroids or scar tissue in the uterus may also affect your risk of miscarriage, but this depends on their size and position.
- Cervical weakness or damage. The cervix is a kind of ‘gateway’ between the uterus and the vagine. It normally dilates (widens) during labour to allow the baby to be born. If the cervix is weak or damaged, it can dilate far too early and lead to second trimester (late) miscarriage.
There are several hormonal conditions that may be connected with miscarriage.
- Polycystic ovarian syndrome (PCOS) is associated with an increased risk of miscarriage. This may be due to increased levels of insulin and testosterone (male hormone) that many women with this condition have, but the relationship is not clear.
- Diabetes that is well-controlled does not increase miscarriage risk. Poorly controlled diabetes may mean a higher chance of miscarriage.
- Thyroid problems that are well controlled do not increase miscarriage risk. Untreated thyroid disease or high levels of thyroid stimulating hormone (TSH) or thyroid antibodies may increase miscarriage risk.
- Prolactin. Abnormal levels of prolactin may increase the risk of miscarriage.
There is no clear evidence to show that immune problems, including raised levels of NK calls, cause or increase the risk of miscarriage. There is a need for more research in this area.
Some serious infections can cause or increase the risk of single miscarriages, but it is not clear if infection plays a role in recurrent miscarriage.
Abnormal DNA in sperm may increase the risk of recurrent miscarriage. Age and lifestyle factors may also play a part.
We talk about tests and treatments here.