A Cardiff hospital has been forced to apologise for flaws in the way it diagnosed miscarriages over many years.
It follows the case of a woman wrongly told she had miscarried nine weeks into her pregnancy after a scan at the University Hospital of Wales.
Emily Wheatley, from Monmouth, went on to have a healthy baby daughter, Ella.
The Public Service Ombudsman for Wales believes flaws in UHW's practices may have gone back as far as 2006. A helpline has been set up for patients.'Real shock'
Ms Wheatley was told during a dating scan she had suffered a silent miscarriage - where there are no symptoms.
She chose to undergo a uterine evacuation at Nevill Hall hospital in Abergavenny, but staff there discovered she was nine weeks pregnant with a healthy foetus.
Ms Wheatley suffers from polycystic ovary syndrome and endometriosis, meaning the chances of her conceiving naturally were "very, very slim".
"To be told that I'd miscarried was a real shock," she said.
"It took a lot to adjust to that after adjusting to the fact that I was pregnant in the first place."
After being told she was indeed still pregnant, she said: "Even though the baby was there clearly on the screen, I couldn't really believe it."
She said the hospital "took away the enjoyment of pregnancy".'Unacceptable mistake'
She added: "It's just unbelievable actually that there are potentially other women out there who have been diagnosed with having a silent miscarriage... and they potentially have got rid of healthy babies. That frightens me."
"Maybe hundreds of babies have been lost because of their decision making, which is unthinkable."
Peter Tyndall, Public Service Ombudsman for Wales, said it was an "unacceptable mistake" which should have been avoided and he has called a review of midwife sonographers' competency.
He said staff, after discovering a silent miscarriage, should have used a different scan to give them a "more accurate picture", but failed to do so.
In a report, he says the health board "failed to implement guidelines issued by the Royal College of Obstetricians and Gynaecologists (RCOG) that were designed to prevent the misdiagnosis of early pregnancy loss" and had been using out-dated guidance for two years or more.
Asked how many other people could have been affected, he replied: "You'll have to ask the health board but clearly there will have been others.
"The health board has set up a helpline for other women who think they may have been affected.
"We'd advise them if they are concerned to contact the helpline."
Mr Tyndall said he thinks Ms Wheatley's case is "unusual and I don't think it's typical", but he said other women may have been similarly affected.
The ombudsman made a series of recommendations, including that the health board issues a written apology and pays the woman £1,500.
The helpline - 0800 952 0244 - was launched on Friday and will remain open from 09:00 to 17:00 daily until 17:00 GMT on 4 November.'Genuinely sorry'
Dr George Findlay, Cardiff and Vale Health Board director for children and women's services, said about 6,000 deliveries are performed each year, and between 600 and 1,200 people have a miscarriage.
End Quote Dr George Findlay Cardiff and Vale Health Board
We let her down and we didn't provide a standard of care that's acceptable to me as a doctor or me as a manager"
"We're saying that about 600 women per year may have a miscarriage that leads to an intervention by us," he said.
"What I don't know right now is what type of scan or number of scans that these patients had and we're happy to look at that on a case by case basis."
Dr Findlay added: "We let her down and we didn't provide a standard of care that's acceptable to me as a doctor or me as a manager."
Cardiff and Vale University Health Board's executive director of nursing, Ruth Walker, offered an "unreserved" apology to Ms Wheatley and said the said the board was "genuinely sorry that it has taken an ombudsman's report for her to receive the answers she deserved".
She said that correct procedures were now followed.
She added the helpline had received 28 calls throughout the day on Friday.
"We were able to immediately reassure eight of those patients regarding their care and will be speaking further with 10 other patients in the coming days.
"There were also 10 calls from patients outside the Cardiff and Vale area which we were able to redirect accordingly."