Around half of early miscarriages happen because the pregnancy had too many, too few or rearranged chromosomes. Here we walk through each type of change, from trisomy to triploidy, and explain why most are one-off chance events rather than something inherited.
Most of these changes arise as an egg or sperm forms, or in the first few cell divisions after fertilisation, and in most cases the parents' own chromosomes are normal. If you have a result to make sense of, our genetic counsellors can talk it through with you.
Overview
When a pregnancy ends in the first few months, the most common reason that can be identified is a problem with its chromosomes. The Royal College of Obstetricians and Gynaecologists (RCOG) puts this at about 1 in 2 early miscarriages, and a large review in The Lancet found a chromosome change in about 6 in 10 miscarriages that were tested.
A pregnancy with the wrong chromosome make-up usually cannot develop as it should, so it stops growing, often before there are any symptoms. This is a one-off chance event in most cases. It is not caused by anything either parent did.
Chromosome changes are a little less common in losses among people who have had several miscarriages: European guidance (ESHRE) reports about 39 in 100, against about 45 in 100 after a single loss. That is one reason other causes are also looked for when losses repeat.
The basics
Each cell normally carries 46 chromosomes, arranged in 23 pairs. One set of 23 comes from the egg and the other set from the sperm. Chromosomes are made of DNA, and the genes along them hold the instructions a pregnancy follows as it grows.
It can help to picture a reference library of 23 numbered volumes, with two copies of every volume on the shelf. A developing pregnancy reads from both copies at once, so the number of copies matters as much as the words inside them. One volume too many, one missing, or a whole extra set can throw the instructions badly out of balance.
Changes can also affect the structure of a volume rather than the count. Pages can be missing (a deletion), printed twice (a duplication), or swapped between volumes (a translocation). If a swap leaves every page present, it is called balanced. If pages end up lost or doubled, it is called unbalanced.
Most errors like these happen when cells divide, either while an egg or sperm is forming or in the first divisions after fertilisation. They are mistakes in copying and sharing out the volumes, not faults in the parents.
Types of change
Laboratories that test pregnancy tissue see the same broad groups of change again and again. Percentages below are shares of abnormal results, not of all miscarriages.
A trisomy means three copies of one chromosome instead of two. It is the most common finding after miscarriage: the RCOG guideline reports trisomies in about 52 in 100 abnormal results, and one large hospital series found about 63 in 100.
Trisomy 16 is the most frequent trisomy, making up about a quarter of trisomies in one large study, followed by trisomy 22 and trisomy 21. Trisomies 21, 18 and 13 are the main ones that can sometimes continue to birth, as Down's, Edwards' and Patau's syndromes. Most other trisomies of the numbered chromosomes end in miscarriage.
Here the pregnancy has a single X chromosome and no second X or Y, written 45,X. It is one of the changes seen most often in miscarriage tissue, at between about 11 and 15 in 100 abnormal results, depending on the study. Studies suggest it often arises when the father's sex chromosome is lost, and it does not become more common as the mother gets older.
A missing copy of one of the numbered chromosomes is rarely found (fewer than 1 in 100 samples in one large series), probably because development stops extremely early. Monosomy X is different: it can occasionally continue, as Turner syndrome.
In triploidy the pregnancy has three complete sets of chromosomes, 69 rather than 46; in tetraploidy it has four. Together these whole-set changes make up roughly 10 to 19 in 100 abnormal results, with triploidy the more common of the two. They arise around the time of fertilisation, for example when two sperm enter one egg, or when the extra set comes from the egg.
When the additional set is the father's, this can mean a partial molar pregnancy. This needs follow-up blood or urine tests for the pregnancy hormone hCG until levels return to normal, usually coordinated by a specialist centre. If the extra set came from the egg, the pregnancy is not molar.
Sometimes every chromosome is present but a section of one is missing or doubled. Large changes can be seen with traditional chromosome analysis. Smaller ones, often called copy number variants (CNVs), need a whole-genome test such as a chromosomal microarray, which may find them in a further 5 to 7 in 100 miscarriages.
Not every small change explains a loss. In one large study, about two-thirds of the small changes found were 'variants of uncertain significance', meaning their effect is not yet known.
Here pieces have moved between chromosomes (a translocation) or been turned round within one (an inversion), and the pregnancy has ended up with too much of one region, too little of another, or both. Structural changes of all kinds make up about 6 to 7 in 100 abnormal results.
This is the group where inheritance matters most. In one hospital series, about 3 in 10 unbalanced rearrangements were traced to a parent carrying a balanced form, and the rest had arisen for the first time. Another series, which tested the parents who agreed to it, found a parental origin more often. That is why parents are offered a blood test when this result appears.
In mosaicism, some cells carry a chromosome change and others do not, because the error happened after fertilisation. One hospital series found mosaicism in about 5 in 100 abnormal results. Mosaic findings can be harder to interpret than a straightforward extra or missing chromosome, so they are best explained by a genetics specialist.
Summary
Figures are approximate shares of abnormal results. They vary with the laboratory method and with who was tested.
| Type | What has happened | Proportion of abnormal results | More common with the mother's age? | Usually inherited? |
|---|---|---|---|---|
| Trisomy | One extra chromosome, 47 in total | About 52 to 63 in 100 | Yes, especially from about 40 (one series found no age link for trisomy 16) | No, apart from rare cases linked to a parent's translocation |
| Monosomy X | One X and no second sex chromosome (45,X) | About 11 to 15 in 100 | No | No |
| Triploidy or tetraploidy | Three or four full sets of chromosomes | About 10 to 19 in 100 | No link found in one large series | No |
| Structural changes | A section missing, doubled or rearranged | About 6 to 7 in 100 | No link found in one large series | Sometimes, when a parent carries a balanced rearrangement |
| Mosaicism | Some cells affected, others not | About 5 in 100 (one series) | Unclear | No |
Based on the RCOG Green-top Guideline No. 17 (2023), Xue et al. (2023), Wu et al. (2021) and Arnadottir et al. (2025). In addition, whole-genome tests may find small missing or extra pieces in a further 5 to 7 in 100 miscarriages.
Sporadic or inherited
Doctors use the word 'sporadic' for a change that happens by chance in one pregnancy. Most chromosome findings after miscarriage are sporadic.
The error happens as an egg or sperm forms, or just after fertilisation. The parents' own chromosomes are normal. In one hospital series of 667 abnormal results, only about 1 to 2 in 100 were an unbalanced rearrangement passed on from a parent's balanced one.
One parent carries a balanced rearrangement. All their genetic material is present, so it does not affect their health, but some of their eggs or sperm carry an unbalanced version. Around 3 to 6 in 100 couples with recurrent miscarriages turn out to have this. Even then, ESHRE estimates only about a third of their losses are caused by the rearrangement.
The outlook is better than many people fear. In a Dutch study of couples tested after two or more miscarriages, 83 in 100 carrier couples went on to have a healthy child, almost the same as non-carriers (84 in 100), although carriers had more miscarriages along the way. Our page on parental chromosome testing explains when it is offered.
Occasionally a trisomy is linked to a Robertsonian translocation in a parent, where one whole chromosome is joined to another. A blood test of the parents' chromosomes would show this, which is one reason a genetics specialist may review an unusual or repeated result.
Age
A person is born with all the eggs they will ever have, and the eggs age along with them. As eggs get older, the chance that one ends up with the wrong number of chromosomes rises. This is a major reason miscarriage becomes more common with age, and why the risk climbs steeply after 40.
Two large hospital series found extra or missing chromosomes significantly more often in miscarriages among women aged 40 or over than in younger groups. In one of them, the overall share with a chromosome change was similar from the twenties to the late thirties. The effect is not the same for every trisomy: in that series, trisomies 21 and 22 were more common from 35, while trisomy 16 was not.
Other types do not follow this pattern. Monosomy X did not rise with the mother's age, and in one series it tended to become less common. In the other series, triploidy, structural changes and the smaller missing or extra pieces were not linked to age. If donor eggs were used, it is the age of the egg donor that matters.
For the overall chance of miscarriage at each age, and why different studies give different numbers, see miscarriage statistics.
Beyond chromosome counts
A normal chromosome result means no chromosome change was found in that pregnancy. It does not rule out every genetic cause, or causes that have nothing to do with genetics.
Looking ahead
Many people find a chromosomal result upsetting and reassuring at the same time. It gives a reason, and the RCOG guideline notes that a miscarriage caused by a chromosome change is linked with a better outlook in the next pregnancy than one where the chromosomes were normal, once age is taken into account. This does not apply when a parent carries a rearrangement.
A trisomy in one miscarriage does not seem to make the next miscarriage more likely to be trisomic: an older study of 273 women with two tested miscarriages found no increase. A later pregnancy may have a slightly higher chance than expected for your age of a trisomy such as Down's syndrome, but the overall chance stays low and screening is available.
If you are planning another pregnancy, pregnancy after a chromosomal miscarriage covers screening and testing options, and understanding your result explains how to read a laboratory report.
Support
UK guidance from the RCOG says chromosome testing of the pregnancy tissue should be offered after a third miscarriage or after any loss from 14 weeks, and it may be offered earlier when there are signs pointing to a chromosome condition. Results usually come back through the team that arranged the test. If a result suggests an inherited rearrangement, you should be referred to a clinical genetics service, where a specialist can talk through what it means for you and your relatives.
If the pregnancy has not yet passed and you are wondering about testing, testing before a miscarriage completes explains what may still be possible, and pregnancy tissue testing covers what happens afterwards. For emotional support, Miscarriage UK and Tommy's both run support lines.
The team that arranged your test is usually the best first contact for questions about it. If you would like more time with a result, our genetic counsellors can also see you online, and we can arrange a chromosome test for parents when one is indicated; the options are set out below.
How we can help
If you want a result explained in more depth, or advice on whether anyone else in the family should be tested, these are the appointments we offer. Genetic counselling and the parental chromosome test can be booked directly, the genetics consultation is arranged by enquiry, and all of them work alongside any NHS care you are already receiving. London Miscarriage Clinic is part of London Pregnancy Clinic, so booking opens on the London Pregnancy Clinic website.
Online, 30 minutes
Go through a chromosome result by video with one of our registered genetic counsellors: what the change was, whether it is likely to have happened by chance, and whether testing either parent would add anything. Partners are welcome to join. A 60-minute appointment gives more time for a complex history.
Only when indicated
A blood test that checks whether either parent carries a balanced rearrangement. It is only advised in particular situations: mainly when testing of the pregnancy found an unbalanced rearrangement, or after repeated miscarriages when no pregnancy tissue could be tested. Results usually take two to three weeks. Testing a couple is £550.
Doctor-led, by enquiry
For a mosaic, unusual or repeated result, or a family history that needs a medical view, Dr Harry Leitch, Consultant in Clinical Genetics, can review your reports and advise on next steps, including whether further testing is worthwhile. Appointments are arranged by enquiry.
FAQs
Often the cause is genetic in the sense that the pregnancy itself had a chromosome change, but that is not the same as inherited. Most of these changes happen by chance in one egg, one sperm or a very early embryo. Only a small minority are linked to a balanced rearrangement carried by a parent, and testing the pregnancy tissue is usually what shows whether that needs checking.
No. Extra chromosomes most often arise as the egg develops, which is why they become more common with age. But monosomy X often follows the loss of the father's sex chromosome, and in a partial molar pregnancy the additional set is paternal, most often because one egg was fertilised by two sperm. Balanced rearrangements can be carried by either partner.
It means the pregnancy had the same extra chromosome that causes these conditions in babies who are born. These trisomies are found in miscarriage tissue as well as in continuing pregnancies; trisomy 21 was the third most common trisomy in two large series. Like most trisomies, they usually happen by chance and become more common as the mother gets older.
Not always. Triploidy is the name for a pregnancy with three complete chromosome sets. When two of the sets come from the father, this may mean a partial molar pregnancy, and the hospital arranges hCG follow-up through a specialist service. When the extra set comes from the egg, it is not a molar pregnancy. Your care team will tell you whether any follow-up is needed.
It means the test found a small section of chromosome that is missing or extra, but its effect is not yet understood, and it may have nothing to do with the miscarriage. In one large study, most of the small changes found were of this kind. Ask for it to be reviewed by a genetics specialist, such as your NHS clinical genetics service or our genetic counsellors, who can say what is known about it and whether testing parents might help.
Chromosome errors happen as cells divide. In most cases they arise by chance, and nothing either parent did is known to cause them. The best-established influence is the age of the egg, which nobody can change.
For what the evidence says about everyday activities and lifestyle, see Was it something I did?.
No. You can book an online genetic counselling appointment directly, without a GP referral. It takes place by secure video link, so you can join from home, and your partner is welcome to join too. It helps to have your laboratory report to hand.
If your test was arranged through the NHS, your NHS team and, where needed, your regional clinical genetics service remain free routes for explaining it.
This page describes general patterns from research and guidelines. It cannot tell you what happened in your own pregnancy, and it does not replace advice from the clinicians caring for you or a personal appointment. If you have heavy bleeding, severe pain, a high temperature or feel faint, contact your early pregnancy unit or NHS 111 now, or call 999 in an emergency.
Contact
The figures and clinical statements on this page are drawn from the sources below. Guidance evolves — always discuss your individual circumstances with a clinician.
