Being told a pregnancy is 'measuring small' or 'not growing as expected' can leave you caught between hope and worry. Sometimes the gap is about dates; sometimes development has slowed or stopped. Here we explain how the two are told apart, why a pregnancy can stop developing and what can be tested.
If you are waiting for a repeat scan, that wait is a normal part of UK care. NICE asks for a second look, usually a scan at least 7 days later, before a miscarriage is confirmed, and notes that waiting does not harm the pregnancy.
Start here
You may have heard that the embryo is not growing, the baby is measuring small or the pregnancy has stopped developing. These phrases mean different things, and each leads to a different next step.
The pregnancy is ongoing but measures behind the date worked out from your last period, often because ovulation happened later than that date assumes. A pregnancy that is much smaller than expected does have a higher chance of not continuing, so a follow-up scan is usually offered rather than a firm answer.
There is a pregnancy sac, perhaps with a small embryo, but no heartbeat yet, and the measurements are below the sizes at which UK guidance allows a diagnosis without a repeat scan. Clinicians call this a pregnancy of uncertain viability. Many continue; others do not. Only a repeat scan can tell.
There is no heartbeat and no further development, confirmed in line with UK guidance, usually by a second scan. This is a missed (or silent) miscarriage. Our missed miscarriage page explains what happens next.
On the scan
Early scans are usually done internally (transvaginally), which gives the clearest view. NICE, which writes guidance for the NHS in England, asks for the heartbeat to be checked first. If there is an embryo but no heartbeat, its length from head to bottom is measured: the crown-rump length (CRL). If no embryo can be seen yet, the sac's average diameter is measured instead.
These measurements are only a few millimetres, so small differences mean less than they seem. In one study, when one sonographer measured an embryo at 6 mm, a second could reasonably record anywhere from about 5.4 to 6.7 mm. NICE also points out that a miscarriage diagnosed from a single scan cannot be guaranteed to be 100% accurate, particularly very early in pregnancy.
The size is then compared with what your dates predict. Dates are usually counted from the first day of your last period, which assumes ovulation about two weeks later. NICE advises against relying on the period date alone to judge whether a heartbeat should already be seen.
Size and dates
A gap between the size on the screen and the size expected from your period is common. The possible reasons include:
Getting an answer
The heartbeat is looked for before anything else. If one is seen, the pregnancy is ongoing, even if it measures small, and the question becomes how it grows from here.
With no heartbeat, one internal scan cannot give an answer if the embryo is under 7 mm or an empty sac measures under 25 mm across on average. NICE then recommends a repeat scan at least 7 days later, and says further scans may be needed.
Even when an embryo is 7 mm or more with no heartbeat, or a sac is 25 mm or more with no embryo, NICE asks for a second opinion on the scan, a repeat scan at least 7 days later, or both. After an abdominal scan alone, the repeat is at least 14 days later.
The key question is whether a heartbeat, or new structures such as a yolk sac or embryo, have appeared. Growth is noted too, but research suggests growth alone is not a safe basis for diagnosing miscarriage, because growth rates in continuing and failing pregnancies can overlap.
The second scan often gives a clear answer, though sometimes another is needed. NICE recommends that you are given a 24-hour contact number to use while you wait.
The evidence
Researchers at seven NHS early pregnancy units followed 2,845 pregnancies that were of uncertain viability at the first scan, and recorded which were still continuing at the 11 to 14 week scan (Preisler and colleagues, BMJ 2015).
Ongoing pregnancies
A heartbeat is a good sign, but it does not settle everything when an embryo is clearly smaller than expected. In a study of 292 pregnancies with a heartbeat and confident period dates, the 41 that later miscarried had measured noticeably smaller at the first scan than those that continued. The researchers concluded that growth restriction this early is linked with the pregnancy later stopping.
That same study is reassuring in one respect: the pregnancies that continued also measured slightly small for their dates, on average. A small gap is common. A larger gap is more likely to lead to a follow-up scan to check how the pregnancy is growing.
If the pregnancy continues, its size will be checked again at later scans. NICE says that waiting for a repeat scan has no harmful effect on how the pregnancy turns out. If you have bleeding that becomes heavy (soaking a pad soon after putting it on), or bleeding with severe tummy pain or shoulder pain, or you feel faint or dizzy, call 999 rather than waiting for the appointment.
Causes
If you are asking why your baby stopped growing, the reason frequently lies within the pregnancy itself, not in anything you did. The NHS says miscarriage is often caused by a problem with the way the pregnancy develops, and the RCOG guideline describes early losses as often the result of random chromosome changes.
Chromosome changes are the most common single cause, accounting for about half of early miscarriages according to the RCOG guideline. Studies of missed miscarriages have sometimes found a higher share. In one, where embryos from 233 missed miscarriages were viewed through a camera ahead of surgical treatment, roughly three-quarters of the pregnancies whose chromosomes could be tested had an abnormal result.
The same research found that in about 18 in 100 cases the chromosomes were normal but the embryo had still not developed normally. The RCOG guideline notes that it is not known whether this reflects the mother's health or genetic changes too small for standard tests to see. It is one reason a normal chromosome result does not always end the search for a cause.
More than one can play a part, and sometimes no cause is found.
An extra copy of a chromosome (a trisomy) is the most frequent finding, followed by one or more extra whole sets (polyploidy) and a missing chromosome (monosomy). These are usually chance events, and errors such as trisomies become more common as the egg-provider gets older. See chromosomal causes.
Sometimes only part of a chromosome is missing or duplicated. Occasionally this comes from a parent who carries a balanced rearrangement without knowing it, which is why parental chromosome tests are offered when pregnancy tissue shows an unbalanced change.
Triploidy means 69 chromosomes rather than 46. When two of the three sets come from the father, it may be a partial molar pregnancy, which can show as a small, growth-restricted embryo with an enlarged placenta. A confirmed partial mole needs follow-up hormone (hCG) tests until levels return to normal.
A change in one gene can stop a pregnancy with normal chromosomes from developing. One 2025 study that sequenced pregnancy losses and their parents estimated that around 1 in 136 of all pregnancies is lost this way; it is a single study, so the figure may change. Standard chromosome tests after miscarriage do not look for these changes.
The early placenta and the lining of the womb have to work together, guided by hormones. Problems with either are linked with miscarriage, but they are harder to test for and are still being researched.
Antiphospholipid syndrome (an immune condition that raises the risk of blood clots), thyroid conditions, poorly controlled diabetes, polyendocrine metabolic ovarian syndrome (PMOS, previously called PCOS) and some differences in the shape of the womb are associated with miscarriage, especially repeated or later losses, as are certain infections. These are usually checked after recurrent miscarriage.
Testing
Genetic tests look at the pregnancy itself, so what is possible depends on where things stand. Our What can I test now? page goes through each situation in more detail, and our genetic counsellors can talk it through with you.
| Where you are | Genetic testing of the pregnancy | What usually happens first |
|---|---|---|
| Waiting for a repeat scan | Not usually. The first step is finding out whether the pregnancy is continuing. | A repeat scan (and sometimes a second opinion), as NICE advises. |
| Heartbeat present but measuring small | Not for size alone. Your team can explain the screening offered later in pregnancy. | Follow-up scans to check growth. |
| Confirmed to have stopped, still in the womb | A blood test looking at placental DNA in your bloodstream may be possible before the pregnancy passes or treatment begins, usually with a scan shortly beforehand. Tissue testing can also be planned. See testing before a miscarriage completes. | Talking through waiting, tablets or surgery with your early pregnancy unit. |
| Tissue has passed or been removed and was kept | Chromosome testing of the tissue, if it reached the laboratory promptly. See pregnancy tissue testing. | Checking whether the tissue was sent fresh, not in preservative. On the NHS, testing is usually offered from a third miscarriage or after a second-trimester loss. |
| Nothing could be tested, or losses keep happening | Testing both parents' chromosomes may be offered in specific situations. See parental chromosome testing. | Wider investigations, covered on our recurrent miscarriage page. |
Blood-based (cell-free DNA) testing after miscarriage is not currently part of standard NHS care; UK research, including work funded by Tommy's, is still evaluating it.
Help and support
Your NHS early pregnancy unit is the usual place for repeat scans and for confirming a diagnosis. It should give you a number to call while you wait, and it can tell you what genetic testing is available locally.
Some people also choose a private scan, for a second opinion or for a repeat scan at a time that suits them. Our fetal medicine team can see you without a referral, and you receive a written report on the day to take to your NHS team. A private scan is not a substitute for urgent NHS care. If a scan confirms that the pregnancy has stopped, our genetic counsellors can talk you through the testing options by video; see how we can help below.
For emotional support, Miscarriage UK has a helpline on 0303 003 6464 and Tommy's has a helpline on 0800 0147 800. Support is available for partners too.
How we can help
These are the appointments that fit most closely with a concern about growth, whether you are still waiting for an answer or a scan has confirmed that the pregnancy has stopped. You can book directly, without a referral, or call or WhatsApp us first if you are not sure which one is right for you. London Miscarriage Clinic is part of London Pregnancy Clinic, so booking opens on the London Pregnancy Clinic website.
6–9 weeks · City or West
An early scan with our fetal medicine team that checks for a heartbeat, takes measurements and dates the pregnancy, with a written report. If you are waiting for a repeat scan, you can book it for at least 7 days after your last one, in line with NICE guidance.
From around 10 weeks · City
Dr Fred Ushakov, a specialist in first-trimester ultrasound and fetal anomalies, scans afresh and forms his own view rather than working from earlier reports, then talks the findings through with you. Worth considering if a diagnosis feels uncertain or you would like it checked before deciding on treatment.
Online, 30 minutes
If a scan confirms that development has stopped, a video appointment with one of our registered genetic counsellors goes through which tests may be possible in your situation and what a result could mean for a future pregnancy. Testing of pregnancy tissue is arranged through counselling rather than booked online.
Not necessarily. Many people ovulate later than the period date assumes, and a small gap is common even in pregnancies that continue. What matters more is whether there is a heartbeat and how the pregnancy grows by the next scan.
After IVF your dates are known more precisely, so your team may look more closely at any gap.
Early growth is fast but not perfectly steady. In a study at four London hospitals of pregnancies whose viability was uncertain, scanned 7 to 14 days apart, embryos in pregnancies that continued grew by about two-thirds of a millimetre a day on average; those in pregnancies that later miscarried grew far less.
Even so, growth rates overlapped between the two groups, especially for the sac, and the researchers concluded that rules based on growth alone could be unsafe. That is why UK practice relies mainly on whether a heartbeat or an embryo appears on the repeat scan, not on a growth target.
Often what looks like catching up is simply a pregnancy that was younger than its dates suggested, growing normally from a later starting point. The repeat scan shows whether there is a heartbeat and whether the size has moved on as expected.
In the large UK study described above, about 87 in 100 pregnancies that showed an embryo with a heartbeat on the repeat scan were still ongoing at the 11 to 14 week scan. A heartbeat with steady growth between scans is reassuring, although no single scan can promise how a pregnancy will go.
It is very unlikely. Exercise, sex and flying are not linked with miscarriage. Research has found a link between persistent, high levels of stress and miscarriage, but stress is hard to study and the link is not well understood; there is no clear evidence that everyday worry causes a loss.
Smoking, alcohol and some medicines are associated with miscarriage being more likely. If you drank a little alcohol before finding out you were pregnant, Tommy's says it is highly unlikely to have caused a loss. The most common reason is a chance change in the pregnancy's own chromosomes.
Not on its own. Slow growth is a reason to look more closely, not a diagnosis, and many pregnancies that measure small are simply younger than their dates suggest. If development does stop, chromosome changes are the most common reason, which is why testing may be worth considering at that point. Our page on understanding results explains what a test can show.
Most people who have a miscarriage go on to have a successful pregnancy. When a loss is caused by a chance chromosome change, the outlook for the next pregnancy tends to be better than when the chromosomes were normal, although age still plays a part.
After more than one loss, or a result suggesting an inherited cause, see our pages on recurrent miscarriage and pregnancy after a chromosomal miscarriage.
Yes. You can book a viability scan (6 to 9 weeks) or a second-opinion scan (from around 10 weeks) directly, online or by phone, with no GP referral. Our scans are carried out by fetal medicine doctors and specialist sonographers, and you receive a written report on the day.
If you are already under the care of an NHS early pregnancy unit, keep those appointments too. A private scan can give you an extra look, but your NHS team will still guide decisions about treatment.
This page explains growth concerns in early pregnancy in general terms. It is not a substitute for advice about your own scan from the clinician who has seen your images and knows your history. Seek help at once if you have heavy bleeding, severe pain, pain in your shoulder or a fever, or you feel faint or dizzy: ring your early pregnancy unit or NHS 111, 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.
