Pregnancy and child

Doctors should 'wait longer' before diagnosing miscarriage

"Doctors are being advised to wait longer before they diagnose a miscarriage," The Guardian reports. 

A new study has found evidence to suggest that women should be given a second ultrasound scan, two weeks after the first, to confirm the diagnosis.

Researchers looked at the specific transvaginal ultrasound scan findings that are used to diagnose miscarriage in early pregnancy. The study aimed to investigate whether the current measurements used, and the delay period between a first and repeat scan, are suitable to diagnose a miscarriage.

The study included almost 3,000 pregnant women who had an early pregnancy scan because of pain, bleeding, severe morning sickness or had previously experienced miscarriage or an ectopic pregnancy

It found that measurements of the developing embryo currently used for diagnosis are appropriate. When all measures are taken into account, no healthy, continuing pregnancies would be wrongly diagnosed as a miscarriage.

However, the study did find that if a repeat scan is needed to confirm miscarriage, there are timing issues to consider. Current protocols run a small risk of coming up with a false-positive result – stating a miscarriage has occurred when the pregnancy is actually viable.

It should be noted that most women are able to have a healthy pregnancy after a miscarriage, even in cases of recurrent miscarriages.

It is likely that the results of the study will be looked at by the bodies that set clinical guidelines regarding pregnancy care, such as the National Institute for Health and Care Excellence and the Royal College of Midwives.

Where did the story come from?

The study was carried out by researchers from a number of hospitals, including Queen Charlotte’s & Chelsea, St Thomas’ and St Mary’s Hospitals. The study was funded by the National Institute for Health Research Biomedical Research Centre, based at Imperial College Healthcare NHS Trust, and Imperial College London.

The study was published in the peer-reviewed British Medical Journal. 

The media coverage primarily relates to the finding of a need for better guidance on when repeat ultrasounds should be performed, so they are not performed too soon after the initial scan.

What kind of research was this?

This was a prospective multicentre cohort study looking at the specific transvaginal ultrasound scan findings used to diagnose miscarriage in early pregnancy.

There has been debate over which are the best measurement cut-offs to use to distinguish between a viable pregnancy (a healthy, developing embryo) and a non-viable one. This includes measuring the diameter of the gestational sac (the fluid sac that surrounds the developing embryo in early pregnancy), or the embryo’s "crown-to-rump" length. Previously, different guideline groups had been using different cut-offs. In 2011, new guidance was issued to update the recommended cut-offs to use.

This study aimed to look at the reliability of changes to guidance on cut-off measurement values for diagnosing a miscarriage.

Its observational design is appropriate for such an investigation, as it does not interfere with the pregnancy in any way or cause unnecessary risk to the baby or mother. 

What did the research involve?

The study included 2,845 pregnant women, mainly from hospital units within London. They attended an early pregnancy ultrasound scan because they had pain, bleeding or severe morning sickness, or to give reassurance following a previous miscarriage or an ectopic pregnancy. 

During early pregnancy (the first 12 weeks) transvaginal ultrasound is usually used, because this is much more reliable for viewing the developing baby in the early stages than the standard abdominal ultrasound used in later stages of pregnancy.

Demographic information was recorded, including:

  • reason for scanning
  • maternal age
  • ethnicity
  • gestational age at first scan (calculated from last menstrual period or embryo transfer date after infertility treatment)

Measurements were taken for:

  • gestational sac diameter
  • presence of a yolk sac (present within the gestational sac and which provides key nutrients to the early developing embryo)
  • embryo crown-to-rump length
  • presence or absence of a heartbeat

All scans were carried out by experienced nurse practitioners, ultrasonographers, and doctors with an interest in the use of ultrasound in early pregnancy.

What were the basic results?

Researchers found that women who eventually had a non-viable pregnancy (i.e. who miscarried) generally presented at a later pregnancy stage and had a higher average gestational sac diameter and crown-to-rump length than viable pregnancies.

The study presents extensive data for different cut-off measures and by different pregnancy outcome, which is too in-depth to go into here. A summary of the main findings is given.

At initial scan, the following factors were 100% reliable for indicating miscarriage:

  • empty gestational sac with average diameter of 25mm or greater
  • embryo with crown-to-rump length of 7mm or more without visible heart activity
  • after 70 days of pregnancy, gestational sac with average diameter of 18mm or greater and without an embryo
  • after 70 days of pregnancy, embryo with crown-to-rump length of 3mm or more without visible heart activity

At repeat scan, the following were 100% reliable for indicating miscarriage:

  • both initial scan and repeat scan at seven days or more showing an embryo without visible heart activity
  • pregnancies without an embryo and average gestational sac diameter of less than 12mm where the mean diameter has not doubled after 14 days or more
  • pregnancies without an embryo and average gestational sac diameter of 12mm or more showing no embryo heartbeat after seven days or more

No embryo heart activity and empty gestational sacs at both initial and repeat scans were very high indicators of a non-viable pregnancy.

The researchers noted that gestational sac size at initial scan should be used to guide timing of the repeat scan. An average gestational sac diameter of less than 10mm at initial scan should have a repeat scan more than two weeks later. Current thinking is that the second scan should be carried out around 7-10 days after the first.

How did the researchers interpret the results?

Researchers conclude that, "Recently changed cut-off values of gestational sac and embryo size defining miscarriage are appropriate and not too conservative, but do not take into account gestational age". 

They go on to recommend that guidance on timing between scans and expected findings on repeat scans continue to be too liberal and that protocols for diagnosis should be reviewed to avoid the risk of terminating viable pregnancies.


This observational study reviewed the reliability of different measurements taken at a transvaginal ultrasound scan to diagnose miscarriage during early pregnancy. 

The recommended cut-off values for gestational sac diameter and embryo crown-to-rump length were changed in 2011 based on a number of reports, with mixed findings suggesting previous ones may have been unreliable.

This study looked at the performance of currently used cut-off values, and found that the current cut-offs used to diagnose miscarriage are reliable. No healthy, continuing pregnancies would be wrongly diagnosed as miscarriage using these values.

However, a finding of note was that if a repeat scan is needed to confirm miscarriage, there are some issues around timing. If there is a gestational sac only, with no embryo present, reliable diagnosis can be harder, and researchers say there should be a wait of two weeks rather than one before carrying out a repeat scan. This reduces the chance of incorrect diagnosis from 2% to 0%. If an embryo is identified at the first scan, then interpretation of miscarriage is more straightforward and timing between scans is less of an issue.

This study has a number of strengths, namely a prospective design and large sample size, with measurements taken by experienced professionals, thereby increasing the certainty of the findings. However, there was no available data for 337 women and this may have influenced results.

Miscarriages are common, can happen for many reasons, and most of the time cannot be prevented. If a woman has experienced previous miscarriages, then she may receive closer care and observation during pregnancy.

Lifestyle factors that are linked to miscarriage and that may help to reduce the risk of miscarriage include not smoking or using illegal drugs, not binge drinking, and, if possible, not drinking alcohol altogether, particularly during the first 12 weeks.

If you have been emotionally affected by a miscarriage, either yours or your partner's, your hospital can offer advice on bereavement counselling and coping with the aftermath.

NHS Attribution