The Duchess of Cambridge has brought the subject into the spotlight. But just what is it, and how should sufferers treat their symptoms?
The truth about morning sickness
The news that the Duchess of Cambridge had been admitted to hospital suffering from the acute form of pregnancy sickness, hyperemesis gravidarum, also referred to as morning sickness, will have had many women wincing in sympathy; anyone who has suffered from pregnancy sickness knows how unpleasant and distressing it can be.
According to Jess Male, a trustee of Pregnancy Sickness Support (www.pregnancysicknesssupport.org.uk), hyperemesis gravidarum - in which severe vomiting can lead to dehydration, weight loss and nutrient deficiency - is relatively rare.
"Although 75 per cent to 80 per cent of pregnant women experience some sort of nausea," she says, "only around one per cent of those are at the higher end of the scale."
Little is known about what causes sickness in pregnancy. It is often attributed to raised levels of the pregnancy hormone, human chorionic gonadotropin (hCG), but studies have been inconclusive as to its true cause.
"There's no real definite idea on the medical front as to why some people get it and some people don't," says Janice Al Najjar, a midwife and the maternity nursing manager at Arab Euro Homes Nursing Services in Abu Dhabi.
"There have been a few studies done that suggest that it might be genetic in origin," she says. "There's some evidence to suggest that if you suffered before you got pregnant with motion sickness, migraine or stomach problems, you might be more prone to getting sickness in pregnancy. Some studies have been done on the flora in the gut and a high percentage of the ladies who suffered from hyperemesis gravidarum had this organism called helicobacter pylori… There are all sorts of evidence out there, and there are multiple studies, but none confirm the same link."
What most pregnant women can agree on is that pregnancy sickness is surrounded by myths, many of which can be unhelpful to sufferers. The most common is the idea that the sickness is confined to the morning.
"Morning sickness is a misnomer," says Al Najjar. "It can last all day. Many ladies have a bout in the morning and again in the evening; it's all-day sickness."
Neither, despite much febrile speculation in the press as to the sex of the royal couple's baby, is there any evidence that pregnancy sickness is an indicator of the baby's gender. "There's a myth that you'll be having a female baby [if you have pregnancy sickness] because oestrogen levels are higher when you have a female, but there's no real evidence to suggest that."
Equally unsupported by evidence is the notion that sickness is a sign that the pregnancy is "strong".
"That's an old wives' tale," says Al Najjar. "I can't think why it denotes a more healthy pregnancy. Some ladies think that if there aren't symptoms and signs that they're not properly pregnant, but hormonal levels are always high in pregnancy."
Most problematic of all is the idea that pregnant women are imagining their symptoms and can simply "pull themselves together" or force themselves to eat or drink.
"Can we please dispel the myth of ginger tea?" pleads Male. "It is helpful to some women, but if someone's suffering terribly, there's no point saying they should have ginger tea. Some at one end of the spectrum can function; others are ill and cannot function. It's not a choice."
Although the d uchess's condition kept her in hospital for three days, pregnant women suffering from pregnancy sickness should not panic. "You should go to your doctor if you are showing signs of dehydration - if you have a dry mouth, are dizzy or faint or aren't keeping fluids down," says Male.
Al Najjar agrees. "If you are pregnant and are throwing up in the morning only, and for the rest of the day you are eating and drinking fine and you are coping, then there's no need to worry."