Too many pregnant women are dying from heart attacks and heart problems because doctors do not expect younger mothers-to-be to suffer, according to a new report.
Experts warned that expectant women should be alert to the symptoms of heart problems - which are the leading causes of death in pregnancy or immediately after giving birth.
Investigators found that in some cases, patients received sub-standard NHS care even though they were showing clear signs of suffering a heart attack or heart failure.
Issues such as obesity, family history of heart disease and a trend towards later motherhood increase the risk of suffering a heart attack or heart problems in pregnancy, the Confidential Enquiry into Maternal Deaths report said.
The new analysis of deaths among pregnant women in the UK found that between 2012 and 2014, 8.5 women per 100,000 died during pregnancy or shortly after giving birth (241 in total).
Overall, there was no statistically significant decrease in the maternal death rate since the last report, covering 2009 to 2011.
More than a quarter of the deaths were from heart problems but only 17% of these women were known to have pre-existing heart trouble.
An in-depth look at 153 women's deaths found poor examples of care.
In one case, a woman went to her GP and hospital five times within two weeks complaining of cough, shortness of breath and problems breathing when lying down.
Despite realising the woman had a fast heart rate, doctors prescribed "multiple courses of antibiotics" to which the woman failed to respond "but no further investigations were done".
Eventually, doctors realised she was suffering heart failure. The baby was delivered by Caesarean but the mother died soon afterwards.
The report said: "This woman presented repeatedly with cardiac symptoms and objective signs. Even when she failed to respond to multiple courses of antibiotics, cardiomyopathy was not considered as a diagnosis."
Another woman in her last trimester of pregnancy complained of such severe chest pain that it was causing her to sit upright in a chair for some of the night.
Hospital doctors ruled out the possibility that the woman was suffering from a blood clot but did not investigate further and discharged her.
"Approximately 36 hours after her discharge she collapsed at home with severe abdominal and chest pain," the report said. Doctors in A&E still failed to perform heart monitoring which would have revealed her problems.
"Despite the women having pain which was so severe she sat in a chair for part of the night, she was discharged without an explanation for the pain," the report said.
"Had an echocardiogram or CT (scan) of the aorta been performed at the time of her initial presentation, it may have been possible to intervene surgically and prevent her death, but this window of opportunity was missed."
The report said there were several other cases where echocardiography "would have revealed the correct diagnosis but it was either not done; not available out of hours; or done but misinterpreted".
It concluded: "This report highlighted many instances when pregnant and postpartum women had clear symptoms and signs of cardiac disease, which were not recognised, often because the diagnosis was simply not considered in a young pregnant woman.
"There was evidence of a focus on excluding, rather than making, a diagnosis in women who presented repeatedly for care."
More broadly, the report also said that the Government's ambition to cut maternal deaths by 50% by 2030 "will be a major challenge for UK health services", given the slow progress so far.
Of all the women who died from 2012 to 2014, 105 of their babies were delivered and survived. They left behind a further 253 children, meaning a total of 358 children have been left without mothers.
Professor Marian Knight, who led the research, said: "Whilst dying from heart disease in pregnancy or after childbirth is uncommon, women need to be aware that they may be at risk, particularly older women.
"It is important to seek advice from your doctor or midwife if you have severe chest pain which spreads to your left arm or back. Being breathless when lying flat is not normal in pregnancy and may also be an indication of heart problems."
Cathy Warwick, chief executive of the Royal College of Midwives (RCM), said: "I want to reassure women that the number of maternal deaths in the UK is very low. However, this report shows that governments have a long way to go in reducing maternal deaths.
"There has been some progress, but not enough and the lack of significant change in the overall maternal death rate is worrying. Behind each statistic is someone who has died and whose death perhaps could have been prevented.
"The number of deaths from cardiac-related problems shows there is clearly a need for maternity services to improve on caring for women with heart-related and other serious illnesses in pregnancy.
"This means caring for those who we know have problems better and improving detection rates when these illnesses develop during pregnancy."
Dr Mike Knapton, associate medical director at the British Heart Foundation, said: "It's a tragedy that heart disease is taking women's lives at what should be one of the happiest times a family can have.
"Maternal deaths have significantly reduced in recent decades, but there are still dozens of maternal deaths per year in the UK.
"Pregnancy and childbirth can put extra strain on the heart, particularly if you have a pre-existing heart condition, and some of these deaths are likely to have been caused by undiagnosed conditions which can cause fatal heart attacks without warning.
"Doctors should be vigilant and consider heart disease when a pregnant woman is experiencing symptoms such as chest pain. These findings are an urgent reminder that heart disease comes in many forms and can strike almost anyone."