Miscarriage Causes: A Complete UK Guide
Why does miscarriage happen?
Miscarriage is the loss of a pregnancy before 24 weeks, and it is far more common than most people realise. Around one in five confirmed pregnancies ends in miscarriage, and the true figure may be higher still because many losses happen before a woman even knows she is pregnant. The vast majority occur in the first 12 weeks.
Understanding why miscarriage happens can help you make sense of what your body did, and what it did not do. The single most important message is that miscarriage is almost never your fault.
The most common cause: chromosomal problems
Around half of all first-trimester miscarriages are caused by a chromosomal abnormality in the embryo. When an egg and sperm combine, they each carry half the chromosomes the baby needs. If the wrong number are passed on, or if the chromosomes do not arrange themselves correctly, the embryo cannot develop properly and the pregnancy ends.
These problems are usually random one-off events. They are not inherited from either parent, and they do not mean there is anything wrong with you or your partner. The risk rises with age: at 25 the chance of miscarriage is around 10%, at 35 it is around 20%, and at 40 it is closer to 40%.
Other causes
Other, less common causes include:
- Antiphospholipid syndrome (APS), a blood clotting disorder that can cut off the blood supply to the developing placenta
- Inherited thrombophilias, which can have a similar effect
- Hormonal problems, including polycystic ovary syndrome (PCOS) and low progesterone
- Thyroid disorders, including both under- and over-active thyroid
- Diabetes that is poorly controlled
- Structural problems in the uterus, such as a septum, fibroids, or a weak cervix
- Infections, including listeria, toxoplasmosis, and certain sexually transmitted infections
- Severe illness or trauma, although this is rare
In around half of cases, no cause is ever found. This can be frustrating, but it also means there is nothing you did to cause the loss.
What raises your risk?
Several factors can increase the chance of miscarriage. These include:
- Age - the most significant factor for both partners
- Being very overweight or underweight
- Smoking, including passive smoking
- High alcohol intake during early pregnancy
- Recreational drug use
- Certain medications, which is why you should always check with a doctor or pharmacist before taking anything in pregnancy
- Previous miscarriage, although most women who have one go on to have a healthy pregnancy next time
- Long-term health conditions such as lupus, kidney disease, or uncontrolled diabetes
What does NOT cause miscarriage?
Many parents search for a reason, and end up blaming themselves. It is important to know what does not cause miscarriage:
- Stress or being upset
- Working, including physical jobs
- Exercise, including running or gym sessions
- Having sex
- Eating spicy food, pineapple, or any other ordinary food
- A single cup of coffee
- A fall or a bump, unless it is severe trauma
- Not "wanting the baby enough"
- Thinking the wrong thing
NICE guidance is clear that these are not causes of miscarriage. Blaming yourself will not help you heal, and it is not based on the evidence.
When to ask for help
Knowledge is power. Understanding what caused your miscarriage - or learning that no cause was found - can help you move forward, but it will not replace the support of people who understand. Reach out when you are ready.