Hundreds of viable wanted pregnancies are being lost to British women each year because of unreliable miscarriage tests, new research suggests.
The precise numbers are unclear but scientists believe there could be 400 errors every year with the potential to result in terminations.
This is more than the estimated 300 cot deaths reported each year in the UK.
Experts have called for more research and improved professional guidelines to minimise the chances of such tragedies occurring.
Pain or bleeding, or a previous history of miscarriage, may prompt doctors to carry out an ultrasound investigation early in pregnancy.
Common practice is to measure the size of the gestational sac and the embryo it contains.
If a sac of a certain size appears empty, or an embryo reaches a certain stage of development with no heartbeat, a diagnosis of miscarriage is made.
When there is doubt about the results, physicians are advised to re-measure the gestational sac seven to 10 days later.
If the sac does not grow during that time, it is assumed a miscarriage has occurred. A termination may then follow.
But according to the new findings, from a series of research papers published in the journal Ultrasound in Obstetrics, the margin of error is so high that some perfectly viable pregnancies are being lost.
One study revealed an up to 20% variation in the size of gestational sacs reported by different clinicians measuring the same pregnancies.
Professor Tom Bourne, from Imperial College London, who led a study of more than 1,000 women suspected of miscarrying, said: “Currently there is a risk that some women seeking reassurance with pain or bleeding in early pregnancy may be told they have had a miscarriage and choose to undergo surgical or medical treatment when the pregnancy is in fact healthy.”
Criteria used to diagnose miscarriage varies. In the UK, one classification under Royal College of Obstetricians and Gynaecologists guidelines is an empty gestational sac measuring 20 millimetres or more. Alternatively, a foetal size of 6mm or more with no heartbeat may also indicate a miscarriage.