Why are miscarriages so common?

Miscarriage is a loss of a pregnancy, and it can be divided into early miscarriage (less than 12 weeks of pregnancy) or late miscarriage (between 12-23 weeks of pregnancy). 20-40% of pregnancies result in a miscarriage, and most commonly occur in the 1st trimester (from day 1 of pregnancy to the 13th week). (1) Many women feel guilt and blame that it may have been something they have done. Understandably, these figures, without context, can be daunting for couples who have had a recent loss, or new couples planning a pregnancy. 

The fate of early pregnancy

Humans are insufficient in reproducing compared to other organisms. The chances of a woman becoming pregnant in each menstrual cycle are approximately 20%, in which only 30% of them result in a live birth. (2)

What about the other 70%?

Figure 1: Pregnancy loss iceberg

30% of pregnancies lead to an implantation failure                   

      • The sperm fertilises the egg in a specific part of your Fallopian tube, known as the ampulla. A few days later, the fertilised egg travels into your uterus to implant itself. 
      • If the fertilised egg can’t attach itself, this leads to a miscarriage. This is a natural fate, the endometrial cells lining the uterus are selective. If they sense through chemical signals that there are abnormalities in the genetic material of the fertilised egg, it may ‘reject’ the implantation to avoid an unsafe pregnancy.
      • Another form of failure of implantation is when the fertilised egg implants itself in the wrong place, which you could read more about here. 

30% of pregnancies result in a ‘chemical miscarriage’ 

      • This occurs when the cells that create the baby and the placenta are absent or abnormal. 
      • This type of miscarriage also occurs early, before you can visually see the baby on ultrasound.

10% of pregnancies lead to a ‘clinical miscarriage’ 

      • This is when a baby in the later stages of pregnancy (where it is now visible on ultrasound) stops developing. 
      • If the miscarriage occurs in the 1st trimester, this is likely due to chromosomal abnormalities or the development of the placenta, which helps transfer the nutrients from the mother to the baby. (3) Chromosomes are blocks that make up your genetic material. 
      • If the miscarriage occurs in the 2nd trimester, a maternal medical condition may contribute. External insults can also cause harm to the baby, such as food poisoning, infection and medications.

As you can see, most (70%) of miscarriages occur before the missed period (before women are aware they are pregnant). It is important to note that the risk of miscarriage does not increase if you previously had a miscarriage. Most women will have the same risk, only 1% of them have recurrent miscarriages. (4)

Can miscarriages be prevented?

Unfortunately, no medical or personal intervention can effectively stop a miscarriage – even if it was detected earlier through an ultrasound scan.*

The most common cause of miscarriage appears to be chromosomal abnormalities, especially in the 1st trimester. Age is one of the greatest contributors to this, a factor that we can’t modify. We are born with all the eggs we will ever have, and with age the quality of our eggs decreases, increasing the chances of abnormalities. A woman under 30 has a 10% risk of miscarrying, compared to 50% in women over 45. (5)

However, several lifestyle risk factors can increase your risk of miscarriage, modifying these can reduce your chances. For example – smoking, alcohol, drugs, obesity and high caffeine. It is important to note that there is no clear evidence that stress increases the risk of miscarriage. 

As mentioned above, the health of the mother can also increase the risk of clinical miscarriages. Here are a few examples (3): 

      • Conditions of the reproductive tract – Abnormal womb structures, fibroids, adhesions, cervical impotence (when the neck of the womb is not strong enough to hold the baby). 
      • Hormonal conditions – diabetes (poorly controlled), thyroid problems, polycystic ovary syndrome. 
      • Pregnancy-related conditions, e.g. gestational hypertension/diabetes/kidney disease.

*Although recently, the National Institute of Clinical Excellence (NICE) has recommended progesterone to pregnant women experiencing bleeding in early pregnancy, or those who have had a previous marriage. (6) Despite the changes in guidelines, greater higher quality evidence is needed to confirm this, and by no means does this prevent every miscarriage.


A miscarriage is unfortunately one of the fates of a pregnancy. Though a ‘natural’ fate, the impacts can be undermined by no means – it is often a traumatic experience with long-term consequences on mental health. If you are struggling to cope with the recent loss of your baby, please contact your local GP services for support/counselling.


  1. Collier JAB, Longmore JM, Amarakone K. Oxford handbook of clinical specialties. Oxford: Oxford University Press; 2014.
  2. ‌Macklon NS. Conception to ongoing pregnancy: the “black box” of early pregnancy loss. Human Reproduction Update. 2002 Jul 1;8(4):333–43.
  3. ‌Miscarriage – Causes [Internet]. nhs.uk. 2017. Available from: https://www.nhs.uk/conditions/miscarriage/causes/
  4. Stirrat GM. Recurrent miscarriage I: definition and epidemiology. The Lancet. 1990 Sep;336(8716):673–5.
  5. ‌Tommy’s – How common is miscarriage? [Internet]. Tommy’s. 2015. Available from: https://www.tommys.org/pregnancy-information/im-pregnant/early-pregnancy/how-common-miscarriage
  6. Wise J. NICE recommends progesterone to prevent early miscarriage. BMJ. 2021 Nov 24;n2896.

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