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Chromosomal & Genetic Causes of Miscarriage

Around half of early miscarriages are caused by the pregnancy having an extra or missing chromosome, or a piece of one lost or rearranged. It is usually a one-off chance event as the egg, sperm or first cells formed, not anything either of you did. Here we explain each type, the part age plays and what can be tested.

If a miscarriage has only just been diagnosed and the pregnancy has not yet passed, some tests work only within a short window. Our what can I test now guide sets out the options.

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The basics

Chromosomes: a library of instruction volumes

Picture the genetic code in each cell as a library of instruction manuals, split into 46 volumes called chromosomes and arranged in 23 pairs. One volume of every pair comes from the egg, the other from the sperm. Twenty-two pairs are numbered, roughly by size; the last pair are the sex chromosomes, XX in a female and XY in a male.

The volumes hold the genes, the instructions that tell cells how to grow. An early pregnancy needs the whole library in exactly the right quantities. With a volume too many or too few, or pages lost or copied twice, some instructions come through too strongly and others too weakly, and development often stops within the first weeks.

This is the main reason early miscarriage is so common. RCOG guidance puts about 1 in 2 early miscarriages down to a chromosome abnormality, and around 6 in 10 tested samples show one (Lancet, 2021). The condition pages we link to below are written mainly for ongoing pregnancies, so their screening sections may not apply after a loss.

Chromosome changes in miscarriage: the key figures

Each figure comes from studies of large groups and cannot predict an individual pregnancy; full references are listed below.

About 1 in 2 Early miscarriages in which the pregnancy had a chromosome abnormality (RCOG, 2023)
About 6 in 10 Tested miscarriage samples showing a chromosome change (Quenby et al., Lancet 2021)
About 1 in 4 Trisomies in miscarriage tissue that involve chromosome 16, often the most frequent single finding (Xue et al., 2023)
3 to 6 in 100 Couples with recurrent miscarriage in which one partner carries a balanced rearrangement; estimates vary (RCOG; NICE CKS)

At a glance

The main kinds of chromosome change

Shares among miscarriages that show a chromosome change, combining the RCOG guideline's pooled figures with two large laboratory series (Wu et al., 2021; Xue et al., 2023) and a review (Papas and Kutteh, 2021). Exact proportions vary with the test used and the group studied.

Type of changeWhat has happenedShare of abnormal resultsLinked to the mother's age?
TrisomyOne chromosome is present three times instead of twiceAbout half to two-thirdsYes, becomes more common as eggs age
Triploidy or tetraploidyA whole extra set of chromosomes: 69 or 92 in total instead of 46About 1 in 10 to 1 in 5No clear link
Monosomy X (45,X)A single X chromosome, with no second X or YAbout 1 in 10 to 1 in 5No; one series found it less often in older women
Structural changePart of a chromosome is missing, doubled or unevenly rearrangedAbout 1 in 20 to 1 in 15No clear link

Smaller changes seen only on a microarray (copy number variants) are counted separately: RCOG guidance says they turn up in another 5 to 7 in 100 miscarriages, and not all of them explain the loss.

Number changes

When there are too many or too few chromosomes

Number changes are the most frequent kind. They usually arise when a chromosome pair fails to separate evenly as an egg or sperm forms, or during the earliest divisions of the embryo.

01

Trisomy: an extra copy

One chromosome is present three times instead of twice. Nearly every chromosome can be affected, but trisomy 16 heads the list at roughly a quarter of trisomies in large series, followed by trisomies 22 and 21. Most trisomies stop development early in pregnancy.

Trisomies 21, 18 and 13 account for most trisomies in later miscarriages, and some of these pregnancies continue. They cause Down's, Edwards' and Patau's syndromes, which NHS screening looks for in ongoing pregnancies; see Down's syndrome, Edwards' syndrome and Patau's syndrome.

02

Monosomy X: a missing sex chromosome

Written 45,X, this means a single X chromosome with no second X or Y. Losing one of the numbered chromosomes almost never allows development to continue, so monosomy X makes up most of the monosomies found after miscarriage (more than 9 in 10 in one large series).

It is not linked to the mother's age and often arises because the sex chromosome from the sperm is lost. Some pregnancies with monosomy X continue, and a baby born with it has Turner syndrome; our sex chromosome conditions page covers ongoing pregnancies.

03

Triploidy and tetraploidy: a whole extra set

Triploidy means three full sets of chromosomes (69); tetraploidy means four (92). Triploidy usually ends a pregnancy early, and in a large 2023 series it showed no link with the mother's age.

If the extra set came from the father, usually because two sperm fertilised one egg, the pregnancy may be a partial molar pregnancy. That needs hCG blood or urine checks through a specialist centre until levels are back to normal. An extra set from the mother is not molar. A second molar pregnancy happens in about 1 in 100; see molar pregnancy.

Structural changes

When part of a chromosome is missing, extra or rearranged

Sometimes the count is right but the contents are not: pages missing, doubled or bound into the wrong volume. These make up roughly 1 in 20 to 1 in 15 abnormal results. A traditional karyotype, read under a microscope, sees changes down to about 10 million DNA letters; a chromosomal microarray can pick up changes as small as about 10,000 to 100,000 letters. The last two cards cover less common findings a report may mention.

Deletion

A section is missing, like pages torn out. Large deletions remove many genes at once and are a recognised cause of miscarriage.

Duplication

A section appears twice, like a chapter printed twice. Extra material upsets the balance of instructions just as missing material does.

Unbalanced rearrangement

Two chromosomes have exchanged pieces, leaving the pregnancy with too much of one section and too little of another. It may be new, or come from a parent who carries a balanced version.

Copy number variants (CNVs)

The umbrella term for a stretch of DNA present in more or fewer copies than usual. Small ones appear only on a microarray and are often of uncertain significance; see microdeletions for screening in ongoing pregnancies.

Mosaicism

A mix of cells with and without a change, often because the error happened in the first cell divisions after fertilisation. About 1 in 20 abnormal results were mosaic in one large series.

Uniparental disomy

Both copies of one chromosome came from the same parent. The count looks normal, but a SNP microarray can pick up many cases. In one series it was found in about 1 in 100 samples tested this way.

Sporadic or inherited

Chance event or inherited? Usually chance

A sporadic change is one that arose by chance in a single egg, sperm or early embryo. Most chromosome changes in miscarriage are like this: the parents' own chromosomes are usually normal, and nothing either of you did caused it. In one series of 667 abnormal results, just 10 (about 1.5 in 100) were unbalanced rearrangements inherited from a parent.

The exception is a balanced rearrangement, where two of a parent's chromosomes have swapped pieces with nothing lost. The carrier is healthy, but some of their eggs or sperm receive an unbalanced share. This applies to roughly 3 to 6 in every 100 couples who have had recurrent miscarriages. The UK guideline reports that most carrier couples still have a healthy child, although further miscarriages are more likely along the way.

So a parental chromosome test is not routine. UK guidance offers it if tissue shows an unbalanced rearrangement, or if tissue from recurrent losses could not be tested. How often an unbalanced result traces back to a carrier parent varies between studies, from about 1 in 3 in one series to about 7 in 10 in another. Rarely, a trisomy is inherited through a Robertsonian translocation, which a parental karyotype would show.

Woman sitting on a sofa with her hands clasped under her chin, looking thoughtful
Most chromosome changes in a pregnancy arise by chance and are not inherited

Age

How age changes the picture, and how it does not

Age shapes some chromosome changes far more than others. Overall rates by age are on our miscarriage statistics page.

Age-related

Rises with age: trisomy

Errors in sharing out chromosomes become more frequent as eggs age, most clearly from around 40 in large laboratory series. With donor eggs, the donor's age is what counts.

Not age-related

Not tied to age: monosomy X

No rise with age; one series found it less often in older women.

Not age-related

Not tied to age: triploidy and structural changes

No link with maternal age in a large 2023 series. A parent's balanced rearrangement is part of their own chromosomes, not a result of getting older.

Miscarriage risk by age: two sets of figures

Partly because trisomy becomes more common as eggs age, the overall chance of miscarriage also rises with age. Pooled Lancet data (2021) give about 12 in 100 at 20 to 29, 14 in 100 at 30 to 34, 18 in 100 at 35 to 39, 37 in 100 at 40 to 44 and 65 in 100 from 45.

The RCOG patient leaflet uses an older Danish study with higher figures for older women: roughly 1 in 4 between 35 and 39, and 1 in 2 between 40 and 44. Different countries, decades and ways of counting losses explain the gap, and neither set predicts any one pregnancy.

Beyond chromosomes

When no chromosome change is found

A normal chromosome result answers one question, not all of them.

  • It tells you that test saw nothing abnormal in that pregnancy's chromosomes. It does not exclude other explanations, whether in the pregnancy itself or in the mother's health.
  • Single-gene changes are far too small for chromosome tests to see. Testing for them after miscarriage is mainly a research tool, not routine NHS care.
  • A 'normal female' (46,XX) result can sometimes come from the mother's tissue instead of the pregnancy's, and SNP microarrays can check for this. Our guide to results explains more.
  • Whatever the chromosome results, UK guidance recommends tests for antiphospholipid syndrome and thyroid function, and a scan of the womb's shape, for anyone with recurrent miscarriage; see recurrent miscarriage.

Looking ahead

What a chromosomal cause can mean for next time

Many people find a chromosomal explanation easier to live with, because it points away from anything they did. It is also, on balance, encouraging: the RCOG guideline links a loss with the wrong number of chromosomes to a better next-pregnancy outlook than a loss whose chromosomes looked normal. Age still counts, and the picture differs if a parent carries a rearrangement.

After a trisomy, the chance of a chromosome condition in a later pregnancy may be slightly above the usual figure for your age, but it usually remains low. NHS combined screening, from around 10 to 14 weeks, is offered to everyone. Our guides to pregnancy after a chromosomal miscarriage and preparing for your next pregnancy cover screening, early scans and diagnostic tests.

Finding the cause

How a chromosomal cause can be looked for

In UK practice, the tissue from a miscarriage is generally offered for chromosome testing from a third loss onwards, or after any loss in the second trimester; before that, it is mainly for losses with features pointing to a chromosome condition. What is possible for you depends largely on whether the pregnancy has passed: see what can I test now.

Pregnancy still in the womb

Blood-based testing

While pregnancy tissue remains in the womb, a maternal blood sample carries placental DNA that a lab can check for extra or missing chromosomes. The result is screening-type, not a diagnosis.

Learn more

After the pregnancy has passed

Pregnancy tissue testing

Fresh tissue can be tested with a rapid check for common changes plus a chromosomal microarray, the NHS approach after recurrent miscarriage.

Learn more

Testing the parents

Parental karyotype

A blood test of both partners, used after an unbalanced tissue result or when tissue testing failed. It cannot explain a sporadic change.

Learn more

London Pregnancy Clinic

Talking it through with our genetics team

Our clinics are in the City and West London, but genetic counselling takes place by video, so there is no need to travel. Within the NHS, referral to clinical genetics is not routine after recurrent early miscarriage but may be advised once testing finds a genetic cause or there is a relevant family history; our services can run alongside that care.

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Genetic counselling

Go through a result, what it means for next time and whether more testing makes sense, with a registered genetic counsellor from our partner Jeen Health.

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Doctor-led

Clinical genetics consultation

Dr Harry Leitch, Consultant in Clinical Genetics, sees people with unexpected or complex findings, such as a rearrangement or a variant of uncertain significance. Enquire to book.

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Only when indicated

Parental karyotype

A chromosome blood test arranged with Jeen Health, for one partner (£290) or both (£550), when a tissue result or your history suggests it would help.

Learn more

Chromosomal causes: your questions

Are miscarriages genetic?

Often, though rarely in the inherited sense. About half of early miscarriages involve a chromosome change in the pregnancy's own DNA, but most of these arose by chance in one egg, sperm or early embryo and do not run in families. Only a small share trace back to a rearrangement carried by a parent.

Is trisomy 16 related to Down's syndrome?

Both are trisomies, but of different chromosomes: Down's syndrome is trisomy 21. Trisomy 16 is the commonest trisomy in miscarriage tissue and stops development early. A trisomy 16 result does not mean the pregnancy had Down's syndrome.

Can a chromosome change start in the sperm rather than the egg?

Yes. Monosomy X often follows loss of the sperm's sex chromosome, and triploidy with an extra paternal set usually comes from two sperm fertilising one egg. Either partner can carry a balanced rearrangement, so both are tested when a parental karyotype is needed. Trisomies more often begin in the egg, which is why they rise with the mother's age.

I am under 35. Why did my pregnancy have a chromosome change?

Chromosome changes happen at every age. Trisomy is less frequent in younger women but still occurs, and monosomy X and triploidy do not appear to be linked to age. At a younger age a chromosome change is still usually chance. After several losses, ask about the tests on our recurrent miscarriage page.

Could IVF have caused the chromosome change?

The evidence is mixed, but large studies have not shown a clear difference. The UK guideline cites similar rates of chromosome changes after fertility treatment and after natural conception (about 57 and 54 in 100), and one large series found 49 and 55 in 100. UK guidance does not routinely recommend embryo testing (PGT-A) for unexplained recurrent miscarriage.

What is a variant of uncertain significance?

A small missing or extra piece of chromosome, seen on a microarray, whose effect is not yet known. It may be unrelated to the miscarriage: in one study, 52 of 77 small CNVs (about two-thirds) fell into this group. A genetics specialist can explain what it means for you and whether further tests would help.

If you would like someone to talk to

A chromosome result can bring relief, fresh grief or both. You can talk it over with Miscarriage UK (The Miscarriage Association) on 0303 003 6464, or ring Tommy's midwives free on 0800 0147 800.

About this information

We write these pages to explain, not to diagnose, and they cannot replace advice from a clinician or genetic counsellor who knows your history. Heavy bleeding, severe pain, a fever or feeling faint need prompt care: ring your early pregnancy unit or NHS 111, or dial 999 in an emergency.

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Sources & clinical references

The figures and clinical statements on this page are drawn from the sources below. Guidance evolves — always discuss your individual circumstances with a clinician.

  1. RCOGPatient information: Recurrent miscarriage2023
  2. RCOG (Regan et al., BJOG)Recurrent Miscarriage: Green-top Guideline No. 172023
  3. The Lancet (Quenby et al.)Miscarriage matters: the epidemiological, physical, psychological, and economic costs of early pregnancy loss2021
  4. Scientific Reports (Xue et al.)Genetic analysis of chorionic villus tissues in early missed abortions2023
  5. Molecular Cytogenetics (Wu et al.)Comprehensive analysis of early pregnancy loss based on cytogenetic findings from a tertiary referral center2021
  6. The Application of Clinical Genetics (Papas & Kutteh)Genetic testing for aneuploidy in patients who have had multiple miscarriages: a review of current literature2021
  7. BMJ Medicine (Joyce et al.)Advances in the diagnosis and early management of gestational trophoblastic disease2022
  8. American Journal of Human Genetics (Warburton et al.)Trisomy recurrence: a reconsideration based on North American data2004
  9. NHS England Genomics Education Programme (GeNotes)Presentation: pregnant woman experiencing recurrent miscarriage2026
  10. NICE Clinical Knowledge SummariesMiscarriage: risk factors2023
  11. GOV.UK (NHS screening programmes)Screening tests for you and your baby: Down's syndrome, Edwards' syndrome and Patau's syndrome2026
  12. MedlinePlus (US National Library of Medicine)How many chromosomes do people have?Accessed October 2026