Two in five (39 per cent) felt their child and partner would be "better off without them" and 16 per cent considered suicide.
An additional 31 per cent admit to having an irrational fear that someone "will steal or harm my baby", making it impossible for them to meet or talk to strangers.
Yet despite 43 per cent of mums suffering these extreme thoughts, 65 per cent of those were never told pregnancy and parenthood could affect their mental health.
As a result, one in five (19 per cent) feared they were "going mad" when they fell ill.
Half (49 per cent) of the mums who suffered were also scared of being "judged negatively".
Some 43 per cent believed others will think they are a "bad parent", while 26 per cent were "ashamed" of being mentally ill.
The stigma is so great that a disturbing 17 per cent of mums who fall ill admitted they thought about self-harm to try to cope, with one in 20 going on to self-harm.
One of the biggest barriers to getting help is the fear your child will be taken into care.
One in five mums (19 per cent) who suffered mental illness after birth reported refusing to access help in case their child was taken away.
Official NHS figures show just ten to 15 per cent of mums experience serious mental health issues.
But as a result, some experts now believe there could be a "hidden epidemic" of maternal mental illness, as 64 per cent of mums who fall ill never try to get a formal diagnosis so are missed by official figures.
Overall, the survey showed the most common mental health ailment suffered by new mums is anxiety, experienced by 68 per cent of women after birth, alongside 48 per cent who undergo insomnia due to worry.
A further 35 per cent battle Post Natal Depression, a third (33 per cent) become agoraphobic and 23 per cent are hit by panic attacks.
On average, women's symptoms lasted three to six months, but 29 per cent felt mentally low for a year or longer.
Women were five times more likely to spot their own symptoms with 69 per cent realising themselves they were ill, compared to just 14 per cent whose partners detected they were unwell.
However, almost two in five mums who become ill (38 per cent) never tell anybody how they feel and just a third confide in their GP (33 per cent).
The most common way mums covered up their feelings was to pretend to be 'fine' when quizzed on how they felt, with 94 per cent admitting they lied about their feelings.
Lack of sleep was seen as the main trigger for mums falling ill, with 55 per cent believing this contributed to their condition.
A further 54 per cent said they felt "emotionally overwhelmed" as a new parent, alongside 39 per cent who admitted they tried to "be the perfect parent".
Two in five (41 per cent) blamed hormonal changes while 14 per cent pinned the blame on "the pressure of living up to others" on social media.
As a result, a resounding 80 per cent of mums want society to be more open about maternal mental health issues and the extreme thoughts mums can have.
Seven in ten (69 per cent) want "society to realise it can happen to anyone" while 55 per cent seek reassurances that their child will not automatically be taken into care if they come forward for treatment.
ChannelMum.com founder Siobhan Freegard said: "If your body is broken after birth, everyone understand 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. You are not different or 'going mad' just because you experience it - but you may need professional care and help.
"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.
"With care, compassion and the right treatment, you can get better quickly, bond with your baby and go on to be the parent you want to be."
ChannelMum.com psychologist Emma Kenny added: "This research has highlighted the plight of large numbers of mums in the UK and evidences that even in 2018 post natal depression is still no closer to being effectively dealt with.
"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.
"We need to address this subject openly, removing the shame and stigma so that mums no longer feel that they have to cope alone."
WHAT IS POSTPARTUM DEPRESSION?
Postpartum depression occurs after the birth of a baby.
It affects up to 20 percent of women in the US and 10 percent in the UK.
This is different from baby blues, which commonly include mood swings, crying spells, anxiety and difficulty sleeping.
This is a more severe, long-lasting form of depression.
There may be problems of bonding with the baby, enjoying motherhood, periods of anger or rage, sadness and crying.
There may be the constant feeling of being overwhelmed or possible thoughts of harming yourself or running away and escaping.
PPD is one disorder under the group of illnesses called perinatal mood and anxiety disorders.
Source: Mayo Clinic and Postpartum Progress
