Generally, they lose around 10 per cent of their bodyweight. And for some, it can be so devastating, they terminate the pregnancy.
Sometimes, the unrelenting vomiting stops at 20 weeks. But in Tetro's case, like many others, she was ill for the entire nine months with both of her daughters.
"It totally caught me off-guard," she says.
"Until I was hospitalised, I didn't realise I was that bad. I didn't even know what hyperemesis was."
For a woman adept at managing a career running a string of successful model and talent agencies and sitting on charitable trust boards, Tetro found she couldn't work "or do anything, frankly".
With her first daughter, she was ill from five weeks into the pregnancy right up until she went into labour at 40 weeks - and was in and out of hospital in between with dehydration. With her second daughter, 18 months later, the biliousness began even earlier.
"With the second one, I managed to stay out of hospital with really good medical care," says Tetro, who is married to former All Black Craig Innes.
"But, to be honest, there was not a single thing that helped. The dehydration could be so disorienting.
"But it's just one of those things. You've got to understand you're not the only one who has it and not let it stop you having another baby. Next time around you know what to expect.
"Plenty of women struggle to fall pregnant or fail to retain their pregnancies, or deliver safely. I can't really complain. Did I feel sorry for myself? At the time, yes. But once I was lucky enough to hold our first baby, I realised the price is worth it."
Feeling queasy is an accepted malady of having a baby, certainly three-quarters of New Zealand women suffer morning sickness in the first trimester of their pregnancies. But some will develop hyperemesis (Latin for "over vomiting"), leaving them so weak they are unable to stand or leave the house.
At Auckland City Hospital, where Sarah Ballard is a midwife, the number of women they see with hyperemesis gravidarum (HG) is around 3 per cent. But the actual rate in New Zealand could be as high as 10 per cent.
"I believe it maybe under-reported, especially in Auckland with its multicultural society," says Ballard, also a lecturer at the School of Midwifery at the Auckland University of Technology.
She's convinced there's a cultural genetic component to the disease. Two New Zealand studies have shown the incidence of HG is significantly increased among Pacific Island women (the latest, a 1995 study of women admitted to Wellington Women's Hospital over five years, showed Samoan women were particularly at risk).
There are also women who "grin and bear it", she says - those who persevere through treatment at home, or those whose doctors overlook it as an accepted symptom of pregnancy.
Yet without medical help, it can be hazardous for both mother and baby - a woman can become so dehydrated she loses crucial vitamins and electrolytes, retarding the baby's growth or bringing on premature labour.
Once upon a time, the risk was fatal.
Before author Charlotte Bronte died in 1855, aged 38 and four months pregnant, she wrote to friends of the illness that besieged her: "Let me speak the plain truth - my sufferings are very great - my nights indescribable - sickness with scarce a reprieve."
While her death certificate may say tuberculosis, biographers believe Bronte died from undiagnosed HG.
As little as 70 years ago, the treatment for women with HG was horrendous. Ballard found women were force-fed or given diluted hydrochloric acid.
Until intravenous fluids were recognised as a critical treatment in the 1940s, the fatality rate for women with hyperemesis was 10 per cent. And in the 1950s, there was the tragedy of thalidomide, a drug designed to treat morning sickness but that, in fact, caused birth defects.
While today's therapies are nowhere near as disturbing or cruel, little has progressed in discovering why some women have morning sickness intolerably worse than others.
Ballard says there hasn't been enough research into the causes.
"There are so many theories but we just don't know why," she says. "It's an utter mystery."
Bex Lipp wishes she had known more when, as a 20-year-old, she was admitted to hospital just four weeks into her first pregnancy.
"I'll never forget it. I was so ill I went to my doctor who said, 'Congratulations, you're pregnant. Go home and celebrate'. And I said, 'but I feel like dying, aren't you going to do something for me?' He said there was nothing he could do and I'd be all right. The next day I was dry retching, so it was straight to hospital."
Lipp found herself in a room with three other women with HG.
"One woman was 32 weeks pregnant and she'd been in since she was six weeks - she had tubes through her nose to put food into her stomach, as well as drips. She was screaming 'just get it out of me, make it stop!' I was so afraid that was going to be me."
After a week, Lipp discharged herself, and decided to have an abortion.
"I was very young and I wouldn't ever do that again, now that I have children and survived through pregnancy. But at that time, I didn't know how I would survive like that for nine months," she says.
The Auckland mum suffered hyper-emesis again right until the birth of both of her sons, Billy and Ollie.
She lost 7kg in the first week she knew she was pregnant with Billy.
"Anything I put near my mouth, I would dry retch," she says.
"All I could manage was little shot glasses of water."
Her heightened senses meant she couldn't stomach the smell of food or even the timber dust her husband, a joiner, brought home from work. So she moved to her mother's house, where she "stayed in bed for months on end".
She soon learned to know when things got worse.
"My lips would peel, so I'd go to the A & E where they knew me. They'd put me on a drip and rehydrate me so I could go home again. I don't cope well in hospitals."
Other danger signs that women should be aware of are heart palpitations, infrequent urinating and intolerance of bright light. Lipp's midwife thought her HG could be related to a vitamin B deficiency (new research shows multivitamins early in pregnancy may help). But the vitamin shots, seasickness bands, anything containing ginger, made no difference. Three different anti-emetic drugs also failed to do their job and arrest the vomiting. She was also concerned what effects the drugs may have on her baby.
"In the end I stopped trying to find a solution. I got sick of people suggesting the same old ginger remedy," she says.
She chose meditation over medication, which gave some relief.
Ballard, who has two children, knows that a woman with HG faces a hopeless dilemma: does she keep feeling terrible or does she take medication that hasn't been proven totally safe?
She's upset by the "grin and bear it mentality" that still exists. "These women are really ill. They may end up in hospital beds with nasogastric tubes down their throats. One woman was vomiting up to 70 times a day," she says.
The scant research that's been done into HG has established certain women are at risk, and it may be due to a string of factors. But the mystery is "whether pregnancy triggers changes in a woman's body that makes her more susceptible, or if a pre-existing condition predisposes a woman to HG when she's pregnant," Ballard wrote in an AUT essay.
If you're prone to seasickness, migraine headaches, have had an eating disorder or your mother had HG, you have a greater chance of hyperemesis. Statistics also show it is more common with girl babies and multiple births.
Research suggests that pregnancy hormones, like human chorionic gonadotrophin (hCG), may not be solely to blame, and it's more likely the fault of the sex hormone, estradiol.
In the case of being predisposed to motion sickness, anti-nausea drugs have been found to give respite for some women.
"Some work it out naturally that it helps to lie down and keep still," Ballard says.
"The trouble is, women who are already mothers can't lie down all day, they have to get on with life."
When medication works, it's great, says Ballard, but frequent readmission to hospital is common.
"Once they're back at home, they're not so vigilant with the medication and, with all the stimuli around the home - like having to cook meals - they're back in again."
One woman helped by the medication was Dr Aimee Neels, now the proud mum of four-month-old daughter Ellie.
Even though she had studied hyperemesis during her medical degree - which she struggled to complete during her pregnancy - Neels admits she had no idea how ruthless the condition could be.
"For the first three or four months I didn't really get out of bed. I was nauseous all day long," she says.
"I wasn't eating anything and drinking very little, and what I could get down would mostly come back up. I worried about the baby because I wasn't taking in enough. I lost so much weight, I didn't get back to my pre-pregnancy weight until right at the end.
"There were a few times I was working at the hospital where I had to excuse myself to run off to the bathroom. Now, when I go back to work [at Hutt Valley Hospital], I really empathise with those women who have hyperemesis."
Neels took three different types of medication until she discovered one that lessened the nausea. But even then, she was sick throughout her labour, and continued to be ill afterwards.
The first-time mum suffered complications during the birth and the side effects of the drugs she was put on were - you guessed it - nausea and vomiting.
Neels' first-hand advice: "Don't put off going to the GP. If you're throwing up a lot, it's important to get on top of it. Make sure you try all the different types of anti-emetic medications. Next time, I'd be more prepared, and have the medication there from the start. I just wish there was some magic cure."
It may seem surprising when these mothers say "next time" - why would they would put themselves through the harrowing experience again, when there's a more than 50 per cent chance they'll suffer HG in future pregnancies?
Says Lipp: "You know that some days will be worse than others, where you think you can't carry on, but you have to hang in there. It feels like an eternity, but as soon as that baby is born and in your arms, it's so quickly forgotten. But as much as I really want another baby, there's still a huge part of me that asks 'could I cope with going through that again?"'
