More than 50% of new mothers suffering from mental health problems feel they cannot raise the issue at postnatal check-ups.
Research conducted by the National Childbirth Trust (NCT) found that 60% did not discuss the issue out of embarrassment and fear the doctor would think they were not fit to look after their child.
A further 24% of new mothers with a mental health problem said there was not time to talk about their concerns during the appointment.
A total of 1,012 women up to the age of two years took part in the research, 35% of whom reported an emotional or mental health problem they wanted to talk about.
Through its "Hidden Half" campaign, the NCT is now calling for more funding for six-week checks to give health professionals the time to give every mother a full appointment.
Currently, women are examined alongside an examination of their baby.
The NCT's Sarah McMullen said: "It's hard for mums to discuss their mental health when appointments are so short and mainly focused on their babies.
"There often isn't enough time to ask them how they're feeling.
"A postnatal appointment dedicated to mothers is long overdue and can give health professionals the time to make mums feel comfortable enough to open up and discuss their worries."
Research by the NCT found that open questions from GPs are more effective than questionnaires in getting mums to open up.
Questions such as "How are you finding being a mum?", "how was it getting everything together this morning?" or "Who's looking after you?" were found to encourage discussion.
Dr Stephanie de Giorgio, a GP based in Kent, said: "The Government needs to fund GPs to be able to offer new mothers a dedicated appointment to give them the time to open up about their problems.
"This would allow doctors to create a supportive atmosphere and ask the right questions to uncover any mental health issues."
A Department of Health and Social Care spokeswoman said: "We are committed to improving mental health support for women after they give birth and ensuring they are able to access the right care close to home.
"We are already investing £365 million in perinatal mental health services, and NHS England is leading a transformation programme to ensure that by 2020/21, at least 30,000 more women each year are able to access specialist mental health care if they need it."
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