Doctors should wait longer before diagnosing early miscarriage to avoid the risk of terminating healthy babies, experts have said.
Second scans should mostly only be carried out only if 14 days have passed since the first one, they said.
Miscarriage is common in the first three months of pregnancy. Pregnant women who experience pain or bleeding in the first trimester are usually referred to an early pregnancy clinic for a scan.
A second scan is often performed around seven days later to confirm whether a pregnancy is viable.
Following this second scan, a decision may be taken to end the pregnancy by surgery or medication, or to let the miscarriage progress naturally.
In the new study, published in the British Medical Journal (BMJ), experts said there was a risk healthy pregnancies could be ended because not enough time has passed between the scans.
Professor Tom Bourne, from Imperial College London, who led the study, said doctors could be confident their diagnosis was correct if scans were 14 days apart.
The current guidance from the National Institute for Health and Care Excellence (Nice) of seven days between scans was associated with a "false positive" rate - where the diagnosis is wrong - approaching 2%, the study found.
Prof Bourne said: "Women should be able to rely on a diagnosis of miscarriage. It's an area of medicine where the highest levels of caution are warranted.
"Just one misdiagnosis of miscarriage is too many.
"We have shown that perhaps people are being brought back too early.
"Most guidelines say if you're uncertain, then come back in seven days, repeat the scan and at that time you should expect to see a heartbeat or, perhaps if you have an empty sac, you should expect to see an embryo.
"Our data says that if you do that you are possibly going to have a false positive diagnosis in a small number of cases.
"We are talking about a small number of cases, but remember there should be no errors over something as important as this.
"What we are saying is, in most cases, you simply wait a little bit longer, perhaps 14 days.
"If you do that then the chance of a false positive is just not there, it's zero per cent."
He said women should feel reassured by the findings but had to accept a longer wait between scans.
They would need to "have slightly lower expectations that they won't always get the answer they want straight away," he said.
The new research is based on a study of 2,845 women who attended early pregnancy clinics across London because of pain, bleeding, severe morning sickness or because they had previously experienced miscarriage or an ectopic pregnancy.
The study also found that pregnancies where the unviable embryo was older - for example more than 10 weeks - on a first scan were more likely to end in miscarriage
The team is now calling on Nice and the Royal College of Obstetrics and Gynaecology (RCOG) to update their guidelines on the issue.
Dr Paul Fogarty, a consultant obstetrician and vice-president of the Royal College of Obstetricians and Gynaecologists, said: "Some women are already advised to wait up to 14 days after the first scan to confirm the diagnosis of miscarriage, particularly if there are doubts about gestational age.
"But the current evidence suggests a minimum of seven days is sufficient to ensure a correct diagnosis in the majority of cases.
"It's essential that both the clinicians and the parents are 100% confident that the pregnancy is not viable before making a diagnosis of miscarriage. If there is any doubt the scans will be repeated.
"This study adds to our knowledge of clinical practice and will be considered when the guidelines are next updated."