Hundreds of pregnant women a year are losing much-wanted babies because of unreliable miscarriage tests, new research suggests.
Scientists say the ultrasound checks are resulting in some women with perfectly healthy pregnancies wrongly being told their baby has died in the womb.
Women would then usually have a termination.
Professor Tom Bourne, from Imperial College London, said 400 viable pregnancies a year could be "misclassified" as a miscarriage.
"These numbers are significant and relate to pregnancies that would be highly likely to reach term," he said.
"I don't think women should be anxious, but I do think we should get it right so we don't make any mistakes."
Prof Bourne led a new study of 1,000 women who were thought to be miscarrying.
If women experience pain or bleeding during pregnancy doctors scan the gestational sac inside the womb. If there is no embryo or the foetus has no heartbeat they diagnose a miscarriage.
But where there is doubt doctors are advised to measure the size of the gestational sac seven to 10 days later. If the sac has not grown, a miscarriage is assumed to have occurred.
However, the researchers warn that there can be a natural variation in the size of gestational sac of up to 20%.
The researchers are now calling for more research and improved medical guidelines to reduce the chances of misdiagnosis.
Dr Tony Falconer, president of the Royal College of Obstetricians and Gynaecologists, said: "Healthcare professionals must receive the best training possible to ensure that they are competent in antenatal screening and diagnoses so that mistakes are avoided."
The findings are published in the journal Ultrasound in Obstetrics.