By Nick Triggle
BBC News, health reporter
Women are being let down by a lack of services for those with postnatal mental health problems, nurses say.
Not all women react well to motherhood
Health visitors that help women with post-natal depression are being cut, the Royal College of Nursing said.
Many areas do not have specialist mother and baby units, so women with severe mental illness end up on general psychiatric wards with their babies.
The government said it was acting to ensure mothers with severe depression were given the help they needed.
The RCN called for appropriate resources to assess, support and treat women suffering from mental illness while pregnant or after the birth.
Up to 15% of mothers experience mild to moderate depression, while up to 5% will develop severe conditions such as psychosis and anxiety disorders.
In the worst cases, mental health problems can lead to women killing themselves and their babies.
The RCN said hundreds of women each year needed inpatient care, but in some regions there were hardly any facilities.
Nurses at the RCN's annual conference in Harrogate said the government must make sure each area had a mental health trusts with a specialist mother and baby unit so they could get the best care.
NHS guidelines say that all women with perinatal depression should have access such units and women must not be separated from their babies.
But a survey for the charity Mind last year found that three quarters have no mother and baby unit and more than one in ten still admitted mothers and babies to general psychiatric wards.
Judith Ring, of the RCN's women's mental health group, said: "There are not enough facilities and without the proper care we are putting these woman and their babies at risk."
She also said women with more minor mental health problems were suffering too.
"We have heard of health visitors being cut and this means those regular visits to women in their homes after birth are not happening as much as they should be.
"Health visitors are not able to spend as much time with women as they like and if these problems are not picked up at an early stage they can become more serious."
Donna Kirwin, chairman of the RCN Midwifery Forum, said: "Women are slipping through the net because of shortages. The subtleties of post-natal depression might not be spotted."
A Department of Health spokesman said: "We are taking action to make sure that they can access the support that they need."
He said standards had been set out detailing how high-risk women could be better identified and supported, and what services were needed.
Shadow health minister Tim Loughton said: "The government must make providing appropriate settings for these extremely vulnerable mothers a priority."
And spokeswoman for the Liberal Democrats said health visitors had a "vital role to play".
"Cuts in their numbers leave an unacceptable gaping hole in maternity care."