Pregnancy loss at any stage is a devastating occurrence, and often leaves us wondering what went wrong? We may never get closure around exactly what caused a miscarriage, however we do know various things that can be contributing factors. Let’s look at 13 possible miscarriage causes, and in our next post we will delve deeper into what you can do to help reduce these risk factors.

For most women, a miscarriage is usually followed by a full term pregnancy, however 5% will go on to have a second miscarriage, and 1% to have 3 or more miscarriages in a row.1

Please note, it does not serve us to appoint blame on ourselves or partners. None of the below are guarantees, just risk factors, and sometimes it is a combination of factors or unknown causes. No matter what, please be gentle with yourself if you have experienced a loss.

Miscarriage Causes

 1. Chromosomal abnormalities.

Most miscarriages are due to a defect in the chromosomes, for example an extra chromosome or a missing chromosome, which is called aneuploidy. When there is an extra chromosome it is called trisomy, with trisomy 16 being the most common abnormality at 21.8%, followed by trisomy 22 at 17.9% and trisomy 21 at 10%.2 Most chromosomal abnormalities are considered sporadic and just an unfortunate error that may not be compatible with life. In our follow up article we will discuss how you can reduce the risk of chromosomal errors occurring. These errors increase with age, which is why we recommend particular preconception care for women over 35.

2. Cigarette smoking.

There is a 1% increased relative risk of miscarriage per cigarette smoked per day in pregnancy.3 A study showed women whose partners smoked during pregnancy were at a 17% increased risk of miscarriage than those with non-smoking partners.4 Another study found that women who smoked before or during pregnancy were at risk of their baby having congenital abnormalities, even when smoking was ceased during pregnancy.5 This shows it is important to quit smoking, for both partners, before conception even takes place.

3. Alcohol consumption.

In the first 5-10 weeks of pregnancy, each week that a mother drinks alcohol results in an incremental increased risk of miscarriage by 8% per week.6 We have also long known about the negative effects of maternal and paternal alcohol consumption during the 3 months prior to conception, which significantly increases the risk of the baby having a congenital heart defect.7 The risk of negative impacts from drinking alcohol appear to be worse for women over the age of 30.8 

4. Caffeine consumption.

Risk of pregnancy loss rises by 19% for every increase in caffeine consumption of 150mg/day, and 8% for every intake of 2 cups of coffee per day.9 It can be hard to determine the quantity of caffeine per caffeinated drink, so this is worth keeping in mind when deciding your own risk. It is worth noting that caffeine crosses the placenta and decreases placental blood flow, affecting foetal growth.10 

5. Obesity.

The risk of miscarriage/pregnancy loss in obese women prior to their first live birth is 25-37% higher than non-obese women.11 Additionally, the higher the BMI, the higher the risk of stillbirth.12 

6. Poorly controlled diabetes or insulin resistance.

Insulin resistance, as seen in PCOS (polycystic ovarian syndrome), is a risk factor for early miscarriage.13 A recent study has determined why insulin resistance causes miscarriages. Insulin is toxic to placenta cells (trophoblasts) in early pregnancy, a time when the placenta is particularly sensitive to insulin.14 

7. Thyroid dysfunction. 

Pregnant women with subclinical hypothyroidism or thyroid antibodies are at risk of miscarriage.15 The optimal level of TSH in pregnancy is 0.2-2.5 mU/L, with miscarriage risk being 1.8 times higher at levels of 4.5-10 mU/L, and 3.95 times higher at levels above 10 mU/L.16 

8. Anatomical or cervical incompetence.

If the cervix has a weakness it can start to open too early, causing a miscarriage. Fibroids in certain positions within the uterus can obstruct the ability for the placenta to correctly attach and grow, causing a miscarriage. Structural abnormalities, such as a septate uterus, can obstruct the ability for a pregnancy to continue growing. In women with recurrent miscarriages of 3 or more, 15% were found to have an anatomical cause of the miscarriages.17 

9. Anti-phospholipid syndrome.

Antiphospholipid antibodies cause miscarriages by preventing embryonic adherence to the uterus, through a process of imbalance in blood clotting.18 Sadly, this syndrome is not usually tested for until a woman has experienced 3 or more miscarriages because it is one of the more rare causes of recurrent miscarriage.

10. Chromosomal abnormalities of either partner, eg balanced translocations.

A balanced translocation is when sections of chromosomes have snapped off and reconnected onto the wrong chromosome. This is called ‘balanced’ when the translocation still allows for normal growth. If either parent has a balanced translocation, they can conceive children with or without translocations, however the ones with translocations may have genetic problems that don’t allow them to continue to grow, leading to a miscarriage. The rates of miscarriage when either parent has a balanced translocation is 20-50%, while 4.5% of all recurrent miscarriages are caused by a parent having a balanced translocation.19

11. Bacterial vaginosis.

A meta analysis confirmed that bacterial vaginosis (BV) is strongly related to miscarriage.20 There is a 2-3 times increased risk of spontaneous miscarriage in women with bacterial vaginosis versus women without.21 Studies did note that BV is more consistent with spontaneous miscarriage in the past 6 months than in recurrent miscarriages.21 

12. Thrombophilias such as Factor V Leiden.

Thrombophilias cause blood to clot more easily, leading to venous thrombosis (blood clots) at the site of implantation of embryos. Thrombophilias have been suggested to be a possible cause of recurrent miscarriage in up to 40-50% of cases.22 

13. Natural killer cells and autoimmune dysfunction.

Natural killer cells are a type of immune cell that are plentiful in the uterus around the time of implantation. Their relationship to miscarriage is an area of contention in medicine, with most doctors suggesting they play no role in miscarriages, while some doctors have described possible links.23 A meta analysis and systematic review of NK cells and miscarriage in 2022 has determined that there is a significantly higher number of uterine NK cells in the uterus of women with recurrent miscarriage and recurrent implantation failure.24 They also found that peripheral NK cell testing was not indicative of uterine NK cell levels and is perhaps not a useful way of determining whether NK cells are affecting pregnancy.24 To summarise what that means – a blood test for NK cells will not be useful in determining whether NK cells are the cause of your miscarriages, it would need to be a uterine biopsy.

I hope the above has helped you to discover any possible areas that may be a factor in your own experiences. If you have a friend who has experienced a miscarriage, you may wish to share this page with them. In the next week we will be releasing the follow up post to this one, with suggestions on how you can reduce the risk of miscarriage.

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