Up to two in five deaths among mothers who die in pregnancy or shortly afterwards could be prevented with better care, a new report suggests.
A detailed examination of 124 maternal deaths found 41% may have been prevented if they had received gold standard care.
The findings prompted researchers to warn all pregnant women not to stop taking medication without seeking expert medical advice.
The latest Confidential Enquiry into Maternal Deaths and Morbidity report states that between 2013 and 2015, for every 100,000 women who gave birth, 8.8 women died during pregnancy or six weeks afterwards.
Two thirds of women who died had pre-existing physical or mental health problems.
Overall, out of the 124 cases assessed, experts deemed that 35% received good care.
In 51 cases (41%), improvements to care which may have made a difference to outcome, the authors found.
Seven of nine deaths from epilepsy may have been prevented if women had seen improvements in their care, researchers from the University of Oxford found.
A number of women who died from epilepsy had stopped their medication early in pregnancy. In some instances this was because either they or their treating doctors were not aware that this could leave them and their unborn babies at increased risk from the effects of seizures, the authors said.
They also called for improvements for care among women with mental health problems.
Professor Marian Knight, who led the research, said: "I cannot over-emphasise to women with known health conditions the importance of seeking specialist advice before they stop or change their medicines in early pregnancy.
"GPs may not always have the expertise to give this advice and it is best to discuss with someone who has experience of managing your specific condition, such as your epilepsy specialist, psychiatrist or physician trained in pregnancy medicine.
"If you are planning a pregnancy, it is important to get this advice before you get pregnant.
"High quality care can clearly prevent complications in pregnancy. In order to continue to reduce the number of women dying in and after pregnancy, we now need to ensure all women can access this high quality care with the appropriate teams of specialists before, during and after pregnancy."
An NHS England spokeswoman said: "Due to significant progress in care, the maternal death rate has fallen by nearly half over the last two decades.
"The NHS is committed to providing safer and more personalised care to new and expectant mums which includes expanding access to specialist care for 30,000 more women each year by 2021, ensuring they get the individual care and support needed."