Current national guidelines on the diagnosis of miscarriage may still be associated with misdiagnoses, and should be reviewed in light of new evidence, suggests a study published in The BMJ today.

The Royal College of Obstetrics and Gynaecologists (RCOG) guidelines recommend a miscarriage diagnosis based on an ultrasound scan measurement of the gestational sac diameter (GSD) and the embryo's crown-rump length (CRL).

Guidelines on the cut off values for these measurements were updated in 2011 following concerns that the older recommendations were based on inadequate evidence and leading to misdiagnoses. These changes have been shown to be safe, according to a new study by Professor Tom Bourne at Imperial College London and colleagues.

But they argue that the guidelines now also need to be updated with new criteria for when the initial scan is inconclusive and a repeat scan is needed, as a result of the embryonic and gestational sac sizes being too small.

A repeat scan is recommended, usually 1-2 weeks later, but this guidance is based on expert opinion rather than evidence. So the team of authors also set out to determine new recommendations for the second scan.

They examined the ultrasound scans of 2,845 women admitted to seven early pregnancy units in the UK during 2011-2013. Women were admitted due to vaginal bleeding, pain, severe morning sickness or a previous miscarriage or ectopic pregnancy.

Based on the examination of these ultrasound scans, the authors make specific recommendations for a repeat scan between 7 and 14 days after the initial scan depending on the size of the GSD, CRL and presence or absence of a heartbeat.

"Guidance on timing between scans and expected findings on repeat scans are still too liberal," explain the authors. "Protocols for miscarriage diagnosis should be reviewed to account for this evidence to avoid misdiagnosis of miscarriage and the risk of terminating viable pregnancies."

In a linked editorial, experts explain that "despite ultrasound technology, we cannot get around the fact that often we still must resort to watching and waiting", adding that this latest study "is an important advance that provides greater certainty".