Media playback is unsupported on your device Media caption Essex mum 'bedbound for six months of pregnancy'

For Hannah Dalton, pregnancy meant not being able to drink fluids for eight months without throwing up, going into hospital 27 times for intravenous drips and living off ice lollies and anti-sickness medication.

Hannah, 30, from Thundersley, Essex, had hyperemesis gravidarum (HG), the severe pregnancy sickness the Duchess of Cambridge experienced during her three pregnancies.

She was bedridden for six months, ended up in a wheelchair and, at her worst, her body started to shut down.

"I seriously questioned was this still worth doing," Hannah says.

"We wanted a bigger family but was there a chance that we would lose me. I thought I was dying."

Killing themselves

With support from her family, Hannah continued with her pregnancy and, in April, gave birth to a girl.

The moment she went into labour, the sickness stopped.

More than 5,000 women from across the UK have shared their experience of HG with BBC News:

Most had considered terminating their pregnancy

One in three had thought of killing themselves

About three-quarters were left with long-term physical and mental health problems, including post-traumatic stress disorder (PTSD) and depression

More than one in three said their experience with their GP had been "poor"

Last year, UK hospitals saw more than 36,000 admissions for pregnant women needing urgent care because of extreme sickness and dehydration.

Hormonal changes

The causes of HG are unknown. There is some evidence it runs in families. And if a woman had HG in a previous pregnancy, she is more likely to have it in the next.

Now, scientists at King's College London and Guy's and St Thomas' Hospital are launching a four-year study - the world's largest - in the hope of finding some answers.

Blood samples and medical histories will be taken from at least 1,000 women admitted to hospital with the most severe HG symptoms and others recruited via the charity Pregnancy Sickness Support.

The study will be looking for genetic links and hormonal changes, in particular a protein, GDF15, produced by the placenta, which affects the part of the brain controlling vomiting and nausea.

'Lifelong consequences'

Consultant obstetrician Prof Catherine Williamson says: "The problem we have is that the treatments aren't good enough.

"Our ambition is to identify genetic causes of this condition so we can tell why women have it and identify those at risk.

"We can then develop new treatments that are much more effective so hopefully there won't be any more women with severe hyperemesis, because we can control it."

What is hyperemesis gravidarum?

HG is very different from morning sickness, which affects about 80% of pregnant women

While morning sickness is unpleasant, women can usually continue with their daily lives and eat and drink normally

But those with HG can vomit more than 50 times a day and feel constantly and severely nauseous, significantly interfering with their daily lives

Complications can include serious vitamin deficiency from the excessive vomiting, significant weight loss, dehydration and malnutrition putting the health of both mother and baby at risk

It can often leave a woman bedridden for months, affecting her longer term mental and physical health

One in 100 of all pregnant women is admitted to hospital because of severe sickness in pregnancy

The British Pregnancy Advisory Service (BPAS) estimates 10% of women with HG terminate their pregnancy

Before IV fluids were introduced, HG was the leading cause of death in early pregnancy

There is evidence to suggest Charlotte Bronte died of HG, in 1855 - her death certificate said tuberculosis but she was four months pregnant and had experienced severe nausea and vomiting

Ever since the thalidomide scandal 50 years ago, there has been concern about taking anti-sickness drugs during pregnancy.

The sedative, which was found to ease nausea and vomiting in expectant mothers, left thousands of babies with severe birth defects.

But most women with HG do end up taking some sort of medication to control the vomiting.

Only one, Xonvea, is permitted in Britain for use in pregnancy - but alternatives, such as cyclizine, prochlorperazine and ondansetron, are also regularly prescribed and considered safe by doctors who treat the condition.

Women may also be given vitamin B6 and B12 or steroids. If these don't work, women may need to be admitted to hospital for treatment including intravenous fluids.

Here are the words of one woman who terminated three pregnancies because of HG. She now has a young child.

"It's your own personal hell that you can't escape from. It's devastating. It completely takes over your life, your family's life, so it would be easier either to just miscarry or die.

"The vomiting and retching was so violent and so intense, I couldn't breathe.

"I couldn't take a breath while I was retching, so I passed out and woke up on the bathroom floor and I thought, 'Oh my God, I can't do this.'

'Ginger biscuits'

"I did have some dark moments.

"I wanted this baby so badly but I felt like it was killing me and ultimately, out of pure desperation, led me to have three terminations.

"I developed PTSD. I had insomnia and nightmares when I could sleep.

"The senior consultant came round and said, 'Have you tried ginger biscuits and salty crackers?' and I was like, 'Oh my God.'

"It's like saying to somebody with a broken leg, 'Have you tried rubbing lavender oil on it?'... because if the senior consultant didn't understand, what hope did I have?"

Caitlin Dean, from Pregnancy Sickness Support, says not treating HG has serious risks.

"Increasingly evidence suggests that, while the actual nausea and vomiting is unlikely to harm the offspring, the complications of HG, such as malnutrition, dehydration and mental ill health, can cause lifelong consequences for both mother and baby," she says.

"There are many wonderful, compassionate doctors out there providing excellent evidence-based care for people with HG but unfortunately there are also doctors who do not recognise the condition, are reluctant to prescribe appropriate treatment or are unaware of the evidence base.

"This leads to a vast amount of unnecessary suffering, costly hospital admissions and, all too often, terminations of otherwise wanted pregnancies.

"In 2019, there is very little excuse not to provide this basic level of care for pregnant women."

'Living hell'

Felicity Collins, from Northamptonshire, was desperate for doctors to prescribe her stronger drugs to help her cope with HG.

She was already in hospital, and 24 hours away from terminating her twin pregnancy, when she was finally given steroids to ease the constant vomiting.

"It was such a dark time," she says.

"It was a decision we made because I knew without those drugs, I couldn't carry on.

"I couldn't eat or drink. Everything made me sick. It was so bad. That's how close it came."

For the next six months, she injected herself daily with steroids, finally giving birth to twin boys, Arthur and Harry, who are now three years old.

Media playback is unsupported on your device Media caption Hyperemesis gravidarum: Woman films video diary for HG awareness

In eight weeks of pregnancy, Laura Anderson lost one stone (6.3kg).

"I dream about eating again and drinking again," she says.

"This illness makes you a shadow of who you were… it's nine months of living hell."

'God forbid'

Laura faces about 20 more weeks of HG before she gives birth.

She says: "I fully intend on getting to the end of this pregnancy with a baby, no matter what it does to my health.

"And when this baby girl is born and the HG has gone, I will spend the rest of my life trying to raise awareness about this awful illness.

"I'm doing it for my daughter, in case she gets it, and God forbid that she does."