Almost half of new mothers are not made aware of potentially fatal conditions which could kill them or their newborn, a poll suggests.
Forty-seven percent of mothers said they were not told about crucial signs and symptoms within 24 hours of giving birth, according to the Netmums survey.
And only a quarter of mothers recalled receiving information about symptoms that could alert them to danger.
Meanwhile, midwives are "frustrated" at not having enough time to spend with mothers and their newborns after birth, the Royal College of Midwives (RCM) said.
New mothers need to be aware of a number of symptoms which could mean they are suffering excess bleeding, infections or blood clots, among other possible complications, the RCM said.
They also need to be aware of the signs that could point towards something being wrong with their babies, a spokeswoman added.
The College's poll of 2,000 midwives, 950 student midwives and 100 maternity support workers found that 36% would like to "do more" for women and their babies.
Almost two-thirds of midwives said the time they spend with a new mother is based on a hospital's "organisational pressures" - only 24% said the time spent is determined by the needs of the mother.
The College said midwives must be trusted to make decisions about the appropriate time spent with a new mother and her baby.
These decisions should be based on need rather than organisational or financial constraints, the RCM said.
Meanwhile, the Netmums survey of 500 women found that most did not know what information they should get from midwives.
The RCM advises that, within the first 24 hours after birth, a mother should be informed of potentially life-threatening conditions both to herself and her baby.
A spokeswoman said the majority of maternal deaths in the UK happen after birth so addressing the concerns around post-natal care is of "paramount importance".
As part of its Pressure Points campaign, the RCM is also calling for more midwives to be employed so women can be offered a better standard of post-natal care.
RCM chief executive Cathy Warwick said: "It is clear that our members are taking the strain of an under-funded and under-resourced post-natal service, a service that, without sufficient means, can lead to harmful consequences on the health of mothers and children that the maternity team struggle to care for.
"We fear that financial belt-tightening and the shortage of midwives, particularly in England, means women leaving maternity units too early and being short-changed when it comes to post-natal visits.
"Midwives want to give better post-natal care, but they can't because there aren't enough midwives and they don't have the resources that they need to give women the care that they need and deserve."
Sally Russell, co-founder of parenting site Netmums, said: "Forewarned is forearmed, so it is deeply worrying that almost half of new mums aren't being informed what to watch for.
"Many conditions are easier to treat if women realise sooner they are at risk of becoming unwell, so it is vital this information gets to them early.
"Pressure on budgets may mean not every family is getting the information they need but this is a false economy.
"If a woman becomes ill, it will cost far more to treat her than the price of passing on this vital information.
"The Government must ensure there are enough midwives to ensure every woman gets the right information at the right time."
Labour public health spokeswoman Luciana Berger said: "High-quality post-natal care is vitally important when it comes to ensuring that mums have the information they need to keep a check on their own well-being and give their new baby the best start in life.
"Yet because the Government has failed to give maternity services sufficient priority too many mothers are being badly let down.
"The Government must ensure that new and expectant mums receive the consistent and high-quality care they need and deserve."
Health minister Dr Dan Poulter said: "As a doctor and a minister I know how important post-natal support is and I am determined to make sure women get the best care before, during and after birth.
"In order to give pregnant women and new mums better care and support, we have increased the number of midwives by over 1,500 since 2010 and are training over 5,000 more.
"The National Institute for Health and Care Excellence has also developed quality standards on post-natal care to improve services for new mums.
"Every woman can now give feedback on their maternity care through the Friends and Family test. This helps hospitals to drive up standards and make sure maternity care is truly focused on what mothers and families want."