In around half of early miscarriages the cause is a chromosomal error that arose at fertilisation. Researchers including a team at Maastricht UMC examined 1,745 losses and found an abnormal karyotype in 50.4 percent. With more sensitive techniques that rose to 67.8 percent.
That error happens spontaneously. Not because of anything you ate, lifted, thought or felt.
And still, nearly every woman rewinds her own weeks. That is human, and in this case almost always unwarranted.
What is the most common cause of a miscarriage?
An error in the number or structure of the embryo's chromosomes. Something goes wrong during cell division, leaving one chromosome too many or too few. Such an embryo cannot develop further, and the pregnancy stops. In the Maastricht analysis this applied to at least half of the losses examined.
One important detail: this is an error that arises at that moment, not something you or your partner pass on. Both parents can have entirely normal chromosomes.
The chance of such an error rises with the age of the egg, and to a lesser extent the sperm. That explains the age curve in miscarriage risk by week.
What other medical causes are there?
Alongside the chromosomal group sits a smaller group with an identifiable medical background. Think of thyroid disease, antiphospholipid syndrome, structural abnormalities of the uterus, and poorly controlled diabetes. Together those explain a minority of cases.
| What the research supports | What the research does not support as a cause |
|---|---|
| Chromosomal error in the embryo | Stress or an argument |
| Thyroid disease | Exercise, lifting or bending |
| Antiphospholipid syndrome | Sex during pregnancy |
| Structural abnormality of the uterus | Flying or driving |
| Poorly controlled diabetes | A drink before you knew |
| Smoking and alcohol (risk-raising) | A bad night's sleep |
The right-hand column is not there to comfort you. It is what the literature says.
Smoking and alcohol sit on the left, as factors that raise risk at group level. Dutch public health body RIVM already provides guidance on both outside of pregnancy. The same caveat applies: raising risk across a group is not the same as causing your miscarriage.
Can stress cause a miscarriage?
There is no convincing evidence that stress causes a miscarriage. The 2021 Lancet analysis does list stress as a population-level risk factor, alongside things like night shifts and air pollution. That is a statistical association across large groups, not an explanation for one pregnancy.
I think this distinction is the most important thing in this article. A risk factor in a table is not proof of blame.
Picture two women of 38 who both had a miscarriage. One had just come through a brutal month, the other a quiet one. For both, the odds are around 50 percent that a chromosomal error was behind it, and their diaries do not move that number.
If stress caused miscarriage the way people assume, hardly any pregnancy in a busy life would make it. That is not what we see.
Can a thyroid problem cause a miscarriage?
A clearly underactive or overactive thyroid is linked in the literature with raised risk. The role of antibodies alone, alongside normal thyroid function, is less clear than was long assumed. The European guideline on recurrent loss is cautious about it in its 2022 update.
That caution follows a large trial in which levothyroxine treatment for women with antibodies and normal thyroid function produced no higher live-birth rate.
What thyroid values do and do not tell you is covered in thyroid and pregnancy: TSH values explained.
Why are you often told no cause was found?
Because in most cases nobody went looking, and because the most common cause is only visible in the tissue itself. After one miscarriage, Dutch care does not usually run an extensive workup. Thuisarts, the patient information service of the Dutch College of General Practitioners (NHG), describes investigation as an option after two or more losses.
That feels unsatisfying when you want an answer. At the same time, searching after one loss would turn up nothing for most women.
What can be investigated after repeated loss is set out in recurrent miscarriage: the tests you can be offered.
What to do with this
If you catch yourself reconstructing that one week, write down what you believe the cause was and hold it against the table above. In most cases your suspicion sits in the right-hand column.
If you are heading into a conversation with your GP or gynaecologist, bring the dates of your pregnancies. That is the information that decides whether investigation makes sense.
The wider picture, including the figures and the tests that can follow, is in miscarriage: symptoms, causes and tests.
Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
References
- Essers R, et al. Prevalence of chromosomal alterations in first-trimester spontaneous pregnancy loss. Nature Medicine. 2023. PMID 37996709
- Quenby S, et al. Miscarriage matters: the epidemiological, physical, psychological, and economic costs of early pregnancy loss. Lancet. 2021. PMID 33915094
- ESHRE Guideline Group on RPL. ESHRE guideline: recurrent pregnancy loss: an update in 2022. Human Reproduction Open. 2023. PMID 36873081
- Thuisarts.nl, patient information on miscarriage and investigation after repeated loss. Dutch College of General Practitioners (NHG)
- RIVM, public health guidance on smoking and alcohol
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