Women are more at risk of severe mental illness after giving birth than at any other time in their lives.
In the worst cases it can lead to postpartum psychosis, also known as puerperal psychosis, a mental illness which affects one in 500 new mothers and can result in suicide or them killing their baby.
BBC Newsnight has spoken to people affected by this devastating but poorly understood condition, which often goes undetected because doctors and midwives can fail to recognise the symptoms.
Mother affected out of the blue
The majority of women who have postpartum psychosis will have no family history of mental illness or experience of it themselves, experts say.
Jo Lyall was one such woman. After her second son, Finlay, was born, Jo suffered a devastating episode. It happened one night, just a few days after leaving hospital:
"I placed his sleeping body down on the bed beside me, and my brain simply snapped," she said. "It felt as though somebody had flicked a switch in my head, and I looked at him and was filled with an urge to kill him."
"I put my hand around his tiny neck, not yet strong enough to hold up his own head, and began to squeeze. I wasn't trying to harm him. I knew I mustn't do that, but I wanted to know if I was capable of it."
Jo knew something was badly wrong, but she was too scared to seek help because she thought her children would be taken away from her.
Without treatment she became sicker and began planning how to kill both herself and her two young sons.
"One day I thought about smothering the boys while they had their lunchtime sleep," she said.
"I had to make sure the boys and dog were dead before I took my own life because I couldn't risk them surviving if I didn't," she added.
Jo made several suicide attempts, but after six months in a secure psychiatric hospital and four years on medication, she is now fully recovered.
She is now campaigning for greater awareness of the symptoms of postpartum psychosis to enable doctors and midwives to offer better treatment to women who are ill.
"I survived, largely due to one consultant and an extraordinary amount of luck," she said. "But women should not have to rely on luck to survive a very treatable condition."
High risk mother needing extra support
Experts do not know the exact cause of postpartum psychosis, although the massive hormonal changes that follow childbirth are believed to play a role, along with genetics.
A proportion of new mothers are more at risk than most - women with bi-polar disorder have a one in two chance of becoming severely ill in the weeks after giving birth.
Shelley Blanchard was in this category. As a result her medical team were keen not only to monitor her physical health as she approached her baby's birth, but her psychological wellbeing too.
Shelley was supported through the final stages of her pregnancy and the early months of motherhood by a team which included Dr Nick Best, a perinatal psychiatrist who specialises in caring for pregnant women and new mothers with mental health problems.
Dr Best was on hand to see her regularly and she also received home visits from the community psychiatric nurse.
Postpartum psychosis can appear very suddenly.
"A person can move from being relatively amenable and understanding of her situation to floridly unwell, psychotic, delusional and paranoid in the space of just two or three days," Dr Best said.
Shelley also started on a course of anti-psychotic drugs on the very evening that she gave birth to baby Oliver.
But a few weeks after the birth Shelley's mood began to drop and she stopped taking the anti-psychotic drugs because they made her drowsy.
"I started getting unpleasant thoughts about Oliver, about wanting to hurt him. They were about throwing him down the stairs or dropping him on purpose," she said.
"I was just so frightened, I didn't want to hurt my boy and I knew somebody had to help me, but the thoughts were getting stronger and more frequent, so I had to tell somebody, I had to get some help."
She contacted her medical team and eventually was admitted to a special unit in Winchester, where mothers and their babies can be kept safe during treatment.
Three months later, both Oliver and Shelley were home again and doing well.
"If I wasn't able to go into the unit, I think that I probably would have ended up taking an overdose, possibly killing myself. I was out of control," Shelley said of that time.
"It was such a dark time, but out of that dark time, I've managed to learn a little bit more about myself. I'm actually feeling really well now, I could actually say that I'm feeling fantastic."
Father who lost wife and child
Dave Emson knows how serious postpartum psychosis can be - when his daughter Freya was just three months old, his wife Daksha stabbed her and then set both the baby and herself on fire in their bedroom.
Daksha died from her burns nearly three weeks later.
"I came home at about half past five or so, and as I got to the front door I smelt a burning smell," Dave said, recalling the day it happened.
"Normally I'd call out 'Daksha, monkey, honey I'm home" and she'd reply that she was here and I'd hear this little babbling brook babbling away, and it was quiet. There was silence.
Daksha had left him a note speaking of her fears that her child was prey to "dark forces" and of her desire to protect Freya at all costs.
Daksha had studied psychiatry and was about to become a consultant when she died.
She had chosen the career in part because she suffered severe depression herself for years, but few people knew about her condition as she was afraid of the stigma it would bring.
The inquiry into her death led to new guidelines in the NHS for the treatment of staff with mental illness.
Dave is now writing a book about her story - to help other people in similar situations:
"Primarily it is for Daksha to speak through me, to actually to speak to her brothers and sisters, people that are suffering, fellow mental health workers, people that are suffering with mental health conditions that you are not alone, you do not need to be alone," he said.
For details of organisations which offer advice and support, go online to bbc.co.uk/actionline or call the BBC Action Line to hear recorded information on 08000 933 193. Lines are open 24 hours and are free from a landline. Mobile operators will charge.