Hearing that a pregnancy is measuring small, or may have stopped growing, is hard to take in, and one scan often cannot give a final answer. Our fetal medicine team explains how growth is checked over time, why development sometimes stops, and what can be tested once the picture is clear.
If you have heavy bleeding, severe pain or feel faint, call 999 or go to A&E. Otherwise your NHS early pregnancy unit can assess you and arrange a repeat scan free of charge.
Available atLPC City (E1)LPC West (SW10)
Understanding the scan
On an early scan, the clinician looks for a heartbeat first. If an embryo is visible, they measure its length from head to bottom, called the crown–rump length (CRL); if not, they measure the pregnancy sac. NICE, which sets guidance for the NHS, recommends this order, using an internal (transvaginal) scan where possible.
The measurements are compared with how far along you are thought to be, usually counted from your last period. Cycle lengths vary, so a pregnancy can be younger than the calendar suggests, and NICE advises that period dates alone should not decide whether a heartbeat ought to be visible yet.
So a small measurement can mean the pregnancy is earlier than expected, developing more slowly, or has stopped. If a heartbeat is seen, the pregnancy had not stopped developing at the time of the scan. 'Stopped growing' only fits when there is no heartbeat and miscarriage has been confirmed using NICE's criteria, which always involve a second opinion, a repeat scan or both.
Three possibilities
Scan reports use different words for these situations. Follow-up scanning is how they are told apart.
The embryo is shorter than expected for your dates, but its heart is beating. The pregnancy may simply be younger than you thought, and a later scan can re-date it. Your clinician may still suggest a follow-up scan to check growth.
A sac with no embryo yet, or a small embryo with no visible heartbeat, below the sizes NICE uses when diagnosing miscarriage. Doctors describe this as a 'pregnancy of uncertain viability'. The safe next step is another scan at least 7 days later.
No heartbeat once the embryo is large enough for one to be expected, confirmed by a second opinion and/or a repeat scan, or repeat scanning shows none has developed. This is a missed miscarriage. If the sac never held an embryo, see an empty pregnancy sac.
How it is assessed
An internal scan looks for a heartbeat, then measures the embryo or, if none is visible, the sac. NICE asks clinicians to explain that one scan cannot be 100% accurate, particularly very early on.
If the embryo measures 7 mm or longer without a heartbeat, or an empty sac is 25 mm or wider, NICE recommends a second opinion and/or another scan a minimum of 7 days on, before miscarriage is diagnosed.
A smaller embryo with no heartbeat, or a smaller empty sac, always needs another scan no sooner than 7 days later, sometimes more than one. If only an abdominal scan was possible, NICE asks for at least 14 days between scans, whatever the measurements. NICE says waiting does not harm the pregnancy.
This looks for a heartbeat not seen before and compares measurements. In a study of uncertain early pregnancies at four London hospitals, the embryo lengthened by roughly 0.67 mm a day on average when the pregnancy went on to continue, against about 0.15 mm a day when it miscarried.
Growth rates overlap between pregnancies that continue and those that do not, especially for the sac, so the researchers judged a diagnosis based on growth alone potentially unsafe. The heartbeat and NICE's size limits come first.
Figures from published studies describe groups of people, not any one pregnancy. Sources are listed at the foot of this page.
Causes
Most of the time, nothing you did or did not do caused it. The most common reason is a chromosome abnormality in the pregnancy itself. Chromosomes carry the instructions for development, and an egg and a sperm should each bring one copy of every chromosome. If the egg or sperm carries an extra or missing chromosome, or the fertilised egg ends up with a whole extra set, development often stops in the first weeks.
The RCOG estimates that about half of early miscarriages happen this way. In a 2023 study from China of tissue from 1,160 early missed miscarriages, where development had stopped but the pregnancy had not come away, around 6 in 10 had a chromosome abnormality on genome-wide chromosome tests, not counting findings of uncertain meaning.
Most of these changes arise by chance as the egg or sperm forms, or in the first cell divisions, and the parents' own chromosomes are usually normal. Egg errors become more common with age as the number and quality of eggs decline, one reason miscarriage becomes more common as women get older.
A 2003 study used a fine camera to look at 233 embryos from missed miscarriages before surgery. About 3 in 4 of those tested had a chromosome abnormality, nearly 1 in 5 had a visible developmental problem despite normal chromosomes, and in about 7 in 100 no problem of either kind was found with the tests available at the time.
Chromosome changes
When a change is found in miscarriage tissue, it is usually one of these. Our guide to chromosomal causes of miscarriage goes into more depth.
The most common finding: about half of abnormal results. Trisomy 16 is the most frequent single one, around a quarter of trisomies in one large study. Trisomies become more common as the egg gets older.
Whole extra sets, such as three instead of two (triploidy): nearly 1 in 5 abnormal results. If the extra set comes from the father it can be a partial molar pregnancy, which needs hCG hormone follow-up until levels return to normal.
Usually monosomy X: one X chromosome instead of two sex chromosomes. Monosomies make up about 15 in 100 abnormal results, and monosomy X does not become more common with the mother's age.
Part of a chromosome is missing, duplicated or rearranged: about 6 or 7 in 100 abnormal results. Occasionally a parent carries a balanced rearrangement, which is when parental chromosome testing becomes relevant.
Beyond chromosomes
When the chromosomes are normal, a cause is harder to pin down, and sometimes none is found. Possibilities include:
Genetic testing
Genetic tests are not used to decide whether a pregnancy that is measuring small will continue; scans over time answer that. Testing becomes relevant once a miscarriage is confirmed, if you would like to understand why.
Timing matters. Without treatment, a missed miscarriage often stays in the womb for days to weeks: in one UK study, about 3 in 10 had passed within a week and 6 in 10 within two weeks. While the pregnancy remains in the womb, a blood-based genetic test that reads placental DNA circulating in your blood may be possible; see testing before a miscarriage completes.
A blood-based result is a screening-type answer, not a diagnosis. Across eight studies it found nearly 4 in 5 of the chromosome changes that tissue testing found; the DNA-counting method used in all of those studies cannot detect triploidy, and some samples give no result. If you have a procedure or collect tissue, pregnancy tissue testing examines the chromosomes directly.
On the NHS, tissue testing is usually offered from the third miscarriage, or after any second-trimester miscarriage. After a first or second loss, our genetic counsellors can help you decide whether a private test is possible and worthwhile. For a step-by-step guide, see what can I test now?
A simplified guide. Your own history and scan findings may change the advice you are given.
| Where you are now | What usually happens next | Genetic testing to discuss |
|---|---|---|
| Measuring small, heartbeat seen | A follow-up scan if your clinician advises one | None at this stage; the pregnancy is ongoing |
| No heartbeat yet, below NICE sizes | Another scan after a gap of at least 7 days (14 days if abdominal only) | None yet; wait for a clear diagnosis |
| Miscarriage confirmed, pregnancy still in the womb | A decision about how the miscarriage is managed; see missed miscarriage | A blood-based test before treatment, or tissue testing afterwards if tissue can be collected |
| Pregnancy tissue passed or removed | Recovery and follow-up care | Tissue testing if suitable tissue is available |
| Repeated losses, nothing tested yet | Investigations for recurrent miscarriage | Possibly parental chromosome tests |
At London Pregnancy Clinic
Your NHS early pregnancy unit can assess you and arrange repeat scans free of charge, and for many people that is all that is needed. We can help if you would like an independent opinion, a convenient appointment or advice on genetic testing. A private scan cannot shorten the 7-day gap NICE recommends between scans.
6–9 weeks
A repeat scan by our fetal medicine doctors and specialist sonographers to look for a heartbeat and growth, best booked at least 7 days after your previous scan.
Uncertain findings
From around 10 weeks, a detailed scan with Dr Fred Ushakov, who has a particular interest in the earliest stages of pregnancy, for an independent view of an uncertain or upsetting result.
Online, 30 minutes
A video appointment with a registered genetic counsellor, through our partner Jeen Health, to discuss whether testing is possible now and which test fits.
Complex results
If a result is unusual or suggests an inherited cause, see Dr Harry Leitch, Consultant in Clinical Genetics. Enquire to arrange an appointment.
Not necessarily. If a heartbeat was seen, the pregnancy is developing and may simply have started later than your dates suggest, especially if your cycles vary. In one study of pregnancies with certain dates, embryos that continued still measured slightly smaller than the reference chart on average, but those that later miscarried had measured clearly smaller. So measuring small does carry a higher chance of miscarriage, which is why a follow-up scan may be suggested.
Because real change takes time to show. A heartbeat too early to see today may be clear a week later, and embryos in continuing pregnancies grow by less than a millimetre a day on average. NICE recommends at least 7 days between scans and says the wait does not harm the pregnancy. If you develop heavy bleeding or pain meanwhile, get seen straight away.
Yes, if the first scan was too early to be sure, which is why NICE asks for a repeat scan first. In the London study above, 473 of 1,060 pregnancies of uncertain viability were still continuing at 11–14 weeks, while the other 587 were not, so the repeat scan matters either way. Once miscarriage has been confirmed using NICE's criteria, though, development does not restart.
This is very unlikely. The most common cause is a chance chromosome change present from the very start. Exercise, sex, ordinary work and everyday lifting do not cause miscarriage, and for everyday stress, research has found no clear evidence that it causes miscarriage. Smoking, alcohol (especially heavier drinking) and a lot of caffeine are linked with a higher risk, but a drink before you knew you were pregnant is very unlikely to explain it.
Your NHS early pregnancy unit will usually book it free of charge, which suits most people. Some choose a private scan for an independent opinion, a convenient time or more time to ask questions. Either way, book it at least 7 days after your last scan. Call us on 020 3687 2939 if you are unsure.
Early on, a sac can be seen before the embryo, so one scan showing an empty sac is not enough on its own. Under 25 mm, NICE asks for another scan no sooner than 7 days later; at 25 mm or more, a second opinion and/or a repeat scan. A London study found that if the sac is still empty on two scans more than 7 days apart, miscarriage is highly likely, whatever the growth. Our guide to an empty pregnancy sac explains more.
Not usually. A scan shows the heartbeat and the size of the embryo or sac, and occasionally features that suggest a molar pregnancy, but it cannot show the pregnancy's chromosomes. Only a genetic test of placental DNA in your blood, or of the pregnancy tissue, can look for a chromosomal cause, and even then a cause is not always found.
The days between scans can be among the hardest. Our miscarriage support page lists charities and helplines, and you are welcome to call us on 020 3687 2939 to talk something through.
This page gives general medical information and cannot replace advice from a clinician who knows your history and has seen your scans. If you have heavy bleeding, severe pain, a fever or feel faint, contact your early pregnancy unit or call NHS 111 straight away, 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.
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