There are "worrying gaps" in mental health services for pregnant women, a charity has warned.
The NSPCC said that women face a "postcode lottery" of services because less than half of mental health trusts in England have specialist services for expectant and new mothers.
The wellbeing of more than one in 10 newborn babies across the country could be improved if all new mothers with mental illness had equal access to good services, a report by the charity suggests.
Mental health problems such as depression, anxiety, post-traumatic stress disorder and schizophrenia can begin or escalate when a woman is pregnant or after her baby is born.
The charity said that many of these illnesses are preventable or treatable but in some areas NHS commissioners are "not giving mums' mental health the priority it deserves".
It is calling on ministers to "fill gaps in services" and to ensure that the mental health of mothers is given the same importance as their physical health.
Peter Wanless, chief executive of the NSPCC, said: "This report clearly shows that with the right services, it is possible to prevent the harm caused by maternal mental illness. But opportunities to help many more families are being missed.
"We have to start treating the mental health of mums and babies with the same importance as their physical health.
"Pregnancy and the first months of a child's life are critical for their future wellbeing and parents naturally play a vital role. If the Government is serious about giving every child the best start in life it must take action to fill the gaps in services."
Dr Ian Jones, vice-chair of the Royal College of Psychiatrists' perinatal section, added: "Maternal mental health remains a neglected area but is of huge importance and has long-lasting impact on the woman herself, her family and wider society.
"This NSPCC report highlights the need for specialist perinatal mental health services and the postcode lottery that characterises current provision. We must work to give women and their families the care they require."
Louise Silverton, director for midwifery at the Royal College of Midwives, said: "There is a real and pressing need to detect and improve the care for pregnant women with mental health problems, throughout their pregnancy and after the birth in particular.
"However, this is going to be very difficult to do without the right number of midwives to cope with the rising demand on maternity services, and the right specialist help for these women. The Government have said that this is a high priority area and we need to see that commitment turn into better services for women."
A Department of Health spokesman said: "No woman should have to cope with mental health issues without the right help, which is why we are making sure the NHS is providing even more support to new mothers.
"We will work with key organisations - including the NSPCC - to see what we can do to help make sure every local NHS organisation is as good as the best in supporting women and families."
He went on: "We have already taken action. Health visitors and midwives have a crucial role in making sure mothers with mental health issues get the right support and we have committed to increase the number of health visitors by 4,200 by 2015.
"There are also nearly 1,400 more midwives since May 2010 and a record 5,000 in training who will all qualify in the next few years.
"Improving mental health services for all those who need them is a top priority for the Government."