Scientists said ultrasound checks are resulting in some women with perfectly healthy pregnancies wrongly being told that their baby died in the womb. Women then usually would 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.
Bourne led a new study of 1,000 women who were thought to be miscarrying.
If women experience pain or bleeding during a pregnancy, doctors scan the gestational sac inside the womb. If there is no embryo or the fetus 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 warned that there can be a natural variation in the size of the gestational sac of up to 20 percent, the study published in the journal Ultrasound in Obstetrics found.
The researchers are now calling for more research and improved medical guidelines to reduce the chances of misdiagnosis.