Sadly, miscarriage is very common. The NHS estimates that among women who know they’re pregnant, one in eight of these pregnancies will miscarry. Many more affect women who don’t know they’re pregnant. If it’s happened to you, you’re probably wondering why. Although it’s not always possible to work out what caused a miscarriage, there are some common reasons they may happen. We’ve rounded them up here. 


What is a miscarriage?

If a baby dies in the womb at any stage up to 23 weeks and six days, this is termed a miscarriage. A loss that happens later than this is called a stillbirth. Even if a miscarriage happens early in pregnancy, it can be extremely distressing. You may have been very excited about being pregnant. A loss can be particularly difficult if it was a first pregnancy, you’d been trying for a baby for a long time or if you’d previously had miscarriages. But all miscarriages are extremely upsetting. And it’s natural you may want to understand why you’ve miscarried. Most women want to make sense of the loss, and to know whether there’s something they can do differently next time. But it’s not always possible to work out why it happened, and the chances are the miscarriage wasn’t connected to anything you did or didn’t do.



The most common causes of miscarriages


Chromosome problems

Chromosomes are the blocks of DNA containing detailed instructions on how cells will develop. Sometimes, for reasons we don’t fully understand, the foetus receives too many – or too few – chromosomes at conception. These faults are thought to cause around half of all early pregnancy losses. This doesn’t necessarily mean there’s a problem with your or your partner’s genes. Miscarriages are usually caused by a one-off fault in either the egg or sperm, or in the way the fertilised egg develops. There’s nothing you can do to prevent it happening but the good news is it’s likely your next pregnancy will be healthy. However, chromosome faults are more common in pregnancy in women in their late thirties or older, which is a reason miscarriage happens more often in this age group. Your partner’s age is also a factor – a pregnancy fathered by an older man is more likely to end in miscarriage too. 


Hormonal reasons

Some women have high levels of luteinising hormone (LH), which your body normally produces to trigger ovulation. This can be linked with polycystic ovary syndrome (PCOS), and has been found to raise risk of miscarriage. Other women who miscarry are found to have low levels of progesterone, which helps maintain a pregnancy, or another hormone called hCG, which keeps levels of progesterone up. You may be offered treatments to help with future pregnancies.


Abnormal antibodies

Antiphospholipid antibody syndrome (APS, also called Hughes syndrome or sticky blood syndrome) can cause blood clots to form in the developing placenta, so it doesn’t grow in the way it should and can lead to miscarriage. You may be offered a test for this if you have recurrent miscarriages – around 15 per cent of women who have multiple miscarriages have been found to have high levels of the problematic antibodies. Treatment can include blood-thinning medicines.


Infections

Mild illnesses like colds don’t cause problems but some infections can raise the risk of miscarriage. These include infections of the vagina or uterus, which may cause later miscarriages, after 14 weeks. Listeria, from eating unpasteurised dairy products, and the STI chlamydia are other infections that can be linked with miscarriage. You can talk to your doctor if you’re worried about these. 


Anatomical issues

Sometimes, problems with the uterus or the cervix may result in miscarriage. One example is a condition called an ‘incompetent cervix’, in which the cervix dilates early in pregnancy, leading to pregnancy loss. The weakness may have been present since you were born, or it may be linked to a later injury or surgery, and there’s usually no way of knowing you’re affected before something goes wrong. However, if it’s diagnosed, doctors can give you a ‘cervical stitch’ to reinforce the cervix during a pregnancy. Problems with the womb itself may also be linked to miscarriage – including large fibroids, and, rarely, an irregular shaped uterus. Surgery can often help prevent future problems.


Lifestyle factors

There are some lifestyle factors that may increase risk of miscarriage, including:

  • Smoking during pregnancy
  • Having a partner who smokes, as it may damage his sperm
  • Drinking alcohol every day, or having more than 14 units a week
  • Eating raw or undercooked meat, either before conceiving or in early pregnancy
  • Drinking a lot of caffeine
  • Taking certain prescription and over-the-counter medicines (talk to your doctor or pharmacist before trying to conceive)

What doesn’t cause miscarriage

Women often worry about what they did ‘wrong’. The following are all things people sometimes think cause miscarriage, but don’t:

  • Exercising
  • Having sex
  • Lifting something heavy
  • Having mixed feelings about a pregnancy or considering a termination
  • Getting pregnant soon after a miscarriage or a birth
  • Working hard
  • Eating spicy food

What can I do to prevent future problems?

It’s understandable you might wonder about this but most of the time, doctors can’t be sure what caused a miscarriage, so it’s not possible to tackle the underlying cause. You’ll normally only undergo tests if you’ve had repeated miscarriages, or if you’re being treated in a private fertility clinic. 

It’s always beneficial to your overall health to look after yourself well – exercising regularly, eating a balanced diet, cutting down on bad habits like too much coffee and alcohol, and getting plenty of sleep will help your wellbeing. But unfortunately, these steps don’t necessarily guarantee a healthy pregnancy. If you’re worried about what might happen next time, speak to your doctor or a specialist organisation such as The Miscarriage Association. They will be able to look at your specific situation and hopefully reassure you. It’s worth bearing in mind the majority of miscarriages are linked with one-off chromosome problems that probably won’t recur. And even if you’re in your early forties and have had more than one miscarriage, statistics show the odds of having a healthy pregnancy are still in your favour. Read more about how to prevent a miscarriage here.