"When you're vomiting 30 to 40 times a day and admitted to hospital, it's a completely different complication of pregnancy," said Rachel Treagust, 28, who suffered from HG, as it is known, with all three of her children.
She told AFP she was "literally bed-bound, in the dark, no TV, no smells or food around. you can't even swallow your own saliva, that's how horrific it is".
Morning sickness -- formally called nausea and vomiting in pregnancy -- affects around 30 percent of women in early pregnancy, according to Britain's Royal College of Obstetricians and Gynaecologists.
But only about one in 200 pregnant women is affected by a severe form known by the Latin term of hyperemesis gravidarum, according to the state-run National Health Service.
Hospitalisation is needed in severe cases to treat dehydration with intravenous fluids for a few days, as it is impossible to keep fluids down.
Treagust suffered in silence with her first child, a boy, but with her second, a girl, she was admitted to hospital several times.
She was put on a drip and given intravenous anti-sickness drugs and steroids, which improved her condition. But she said it wasn't long before she would get ill once again and have to go back into hospital.
"It's like a cycle," she said. Her condition generally improved four to five months into her pregnancies, but it wasn't until she gave birth that it completely cleared up.
"The first time I didn't understand the condition. I didn't know that I needed to go to a doctor so I just laid in bed for a couple of months, sipping water. I tried to get by," Treagust said.
By the time she was pregnant with her third child, another boy, she had found the right treatment and a support network in the form of a dedicated charity, Pregnancy Sickness Support, for which she now works.
The charity has seen visitors to its website swell since Catherine, the Duchess of Cambridge, was admitted to hospital on Monday in the very early stages of pregnancy suffering from HG.
"Of course it's unfortunate and sad that the duchess is suffering from this condition but at the same time for the women who are suffering as well it has truly highlighted the situation," Treagust said.
She said one of the worst things was lack of knowledge about the illness.
"It was scary. Not only do you feel so unwell, but you feel quite isolated as people say, 'get up and about, get some fresh air, you'll feel fine. Have some ginger biscuits'. But none of that works," she said.
Hormonal changes in the first 12 weeks are considered the likely cause of morning sickness, and the symptoms can occur day or night.
"Symptoms (of hyperemesis gravidarum) can include persistent vomiting, dehydration, tiredness and dizziness," said Daghni Rajasingam, consultant obstetrician and spokeswoman for the Royal College.
"The condition usually subsides by week 12 of pregnancy, and with early diagnosis and treatment there is no reason why we shouldn't expect a healthy pregnancy," she added.
A review published by the college's journal found that in severe cases, morning sickness can profoundly affect a woman's quality of life, causing "feelings of depression, difficulties between partners and concern for the health of the unborn child."
But there are several "safe, effective" medical options for dealing with it, including antihistamines as well as, in relatively small doses, vitamin B6, the society said.