There is a risk healthy pregnancies could be ended because not enough time has passed between scans, experts say.

Doctors should wait longer before diagnosing early miscarriage to avoid the risk of terminating healthy babies, UK experts have said.

Second scans should mostly be carried out only if 14 days have passed since the first one, they say.

Pregnant women who experience pain or bleeding in the first trimester of pregnancy are usually referred to an early pregnancy clinic for a scan.

A second scan is often performed about 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, experts said there was a risk healthy pregnancies could be ended because not enough time had passed between the scans.

Study leader Professor Tom Bourne, from Imperial College London, said: "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."

Women 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 2845 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.

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."