Almost half of all new mums suffer frightening thoughts or hallucinations after giving birth - but keep quiet for fear of being judged or having their baby taken away.
Researchers carried out a detailed study into the state of mind of British mums after giving birth and found an alarming number experienced the unsettling thoughts.
A fear that something dreadful will happen to their baby is the most common, with 62 per cent of those who suffered the hallucinations vividly feeling this.
Forty-four per cent worried about their baby dying, 13 per cent imagined killing or harming their own child, while one in 100 were convinced their mother-in-law was trying to take their baby.
The study by parenting site ChannelMum.com also found others had visions of their baby being taken by strangers, eaten by a crocodile or snatched by the Grim Reaper.
Worryingly, 39 per cent felt their child and partner would be better off without them and 16 per cent even considered suicide.
Official NHS figures show just 10 to 15 per cent of mums experience serious mental health issues.
But experts believe there could be a 'hidden epidemic' of maternal mental illness, as 64 per cent of those who fall ill never try to get a formal diagnosis so are missed by official figures.
ChannelMum.com founder Siobhan Freegard said: "If your body is broken after birth, everyone understands and supports you, but when your mind is broken, mums still feel they have to keep it hidden.
"These thoughts are disturbing and terrifying - but very often they are part of becoming a mum.
"We need to talk about it, normalise it and make mums realise they are not alone."
The study of 1,670 mums found 65 per cent of those who experienced the awful thoughts weren't told pregnancy and parenthood could affect their mental health.
As a result, one in five feared they were 'going mad' when they fell ill, while 49 per cent were scared of being 'judged negatively'.
Others worried they would be labelled a bad parent (43%) while 26 per cent were 'ashamed' of being mentally ill.
The stigma is so great that a disturbing 17 per cent of mums who fell ill admit they thought about self-harm to try to cope, with one in 20 going through with it.
But 19 per cent of mums who suffered mental illness after birth refused to access help in case their child was taken into care.
Siobhan Freegard said: "I suffered from post natal depression and visualised extreme images after the birth of my first child then tried to cover it up, so I know exactly what women are going through when they hide it.
"There is so much pressure to cope and be the perfect parent that when you are crying inside while everyone around you is smiling, it makes you feel you are failing your baby and your whole family.
"Mums need to know they will get the right support and their baby won't be taken away."
ChannelMum.com psychologist Emma Kenny added: "Good maternal mental health is something that every woman deserves.
"But due to the lack of knowledge and support this simply isn't the case, leaving women to deal with some of the most terrifying and debilitating feelings at a time when they should be enjoying their positive new beginnings."
EXTREME THOUGHTS MOTHERS HAVE EXPERIENCED: "I believed my baby hated me and was trying to kill me""I planned to jump off the balcony with my baby""I thought my flat was haunted and would stay outside from dawn til dusk until my husband got home""I thought my baby would die if I didn't wash up before my microwave pinged""I saw the Grim Reaper outside my bedroom door""I thought my twins weren't mine""I thought that my mother in law was planning to take my baby""I thought that my baby would die because of germs. So I made everyone disinfect themselves before they could touch him. Someone touched my pram in a supermarket once and I couldn't move due to fear of contamination. I stood in the cleaning isle vigorously disinfecting my pram.""I believed if I went to sleep, someone would break in and smother us all to death""I wanted to throw my daughter out of the window""I thought a crocodile was trying to eat my baby""I wanted to drive the car into a wall and kill my baby and myself""I cut all my hair off as I thought my baby was eating it and making himself ill""I thought everyone would be better off without me""I imagined an iron melting into my baby's face""Just blackness, despair like being trapped in thick mud"