New parents are too afraid to seek help to cope with feelings of depression and anxiety, the Royal College of Nursing (RCN) has said.
Its poll of 2,000 new mothers and fathers found 41% experienced anxiety, depression or another mental health issue during pregnancy or after the birth of their first child.
Some 53% of new mothers suffered depression or anxiety over the same period, while 27% of new fathers said they had similar feelings.
Fewer than half (46%) of new parents considered seeking help from a health professional. Of those who did not, 26% said they had felt too afraid, while 11% were unaware support was available.
Most relied on their partner or other relatives for help.
Carmel Bagness, professional lead for midwifery and women's health at the RCN, said: "Too many parents worry that going through depression or anxiety means they will be deemed unfit parents, and this can be a hugely damaging - and incorrect - assumption which is putting lives at risk and preventing people getting the support they deserve.
"Too many women and men are suffering in silence because of outdated stigmas. Too often, attitudes towards mental health are not fit for the 21st century.
"Midwives and nurses know how important openness and understanding is when it comes to mental health.
"Their employers should now ensure they have the training, and the time, to treat the mental health of mothers and fathers as well as they treat their physical health."
Clare Dolman, acting vice chair of the Maternal Mental Health Alliance, said: "As a mother who experienced mental illness after the birth of my daughter, I am very aware of how frightening and isolating an experience it can be - and how much it can affect fathers too.
"I have met many women in a similar situation and the vast majority of them recover very well and are excellent parents, but they need the understanding and support of all those around them, not just family and friends but health professionals too."