More than half of all pregnant women suffer with morning sickness. But rather than dreading it, new research suggests they should welcome it

It's unpleasant and can happen almost every day during a pregnancy, but new research suggests it really is a way of keeping mother and baby alive and is healthy. Although morning sickness affects more than half of pregnant women, no one knows for sure why it occurs or even why some women suffer while others do not.

It's unpleasant and can happen almost every day during a pregnancy, but new research suggests it really is a way of keeping mother and baby alive and is healthy. Although morning sickness affects more than half of pregnant women, no one knows for sure why it occurs or even why some women suffer while others do not.

One theory is that it is a side effect of the hormonal changes in the body, but scientists are now challenging that idea. They argue that such an explanation doesn't fit the bill because over years of evolution, nature would have designed-out such an unpleasant and seemingly unnecessary event.

The new research, the first of its kind, suggests that morning sickness plays a key role in some women to protect both them and their child from toxins in food. Moreover, say the researchers, any attempt to treat morning sickness could have a detrimental effect on mother-to-be or the growing foetus.

Morning sickness is usually suffered in the first three months of pregnancy, and despite its name it can occur at any time during the day, although it is most common in the mornings. Not only is it unpleasant, it can result in weight loss and over a prolonged period the acid in the vomit can cause the enamel of the teeth to break-up.

Several possible explanations have been put forward for morning sickness. One theory is that it is the fall-out from a genetic tussle between mother and the growing foetus, while another suggests it is a way of communicating to males that the woman is pregnant. Another, Freudian explanation, is that it is in some way connected with people not wanting to be pregnant.

But one of the most popular explanations for morning sickness has been that it is simply a side effect of the gestational, hormonal changes that occur in a mother-to-be's body after conception.

"Morning sickness, like the altered sense of taste and smell is a reaction of the digestive system to the hormones in the mother's bloodstream," says Adriana Hunter, author of The Queen Charlotte's Hospital Guide to Pregnancy and Birth.

"The reason why it has been thought to be hormonal is that it is at its worst in the first 12 weeks and that is the time when the pregnancy hormone that makes the pregnancy test positive is at its highest and then starts coming back down again. It has always been assumed it was an unwanted affect of this hormone," says Professor James Drife, Leeds University professor of obstetrics and gynaecology.

But another theory has been gaining ground, and a new report from evolutionary biologists at Cornell University says morning sickness is a mechanism nature has evolved to protect both mother and foetus from illnesses caused by food and from toxins that could deform the growing foetus.

It is estimated that around two to six per cent of infants have some kind of birth defect, ranging from deformed limbs to partial deafness. Although many are blamed on any one of a number of genetic syndromes or pharmaceuticals taken by the mother during pregnancy, two thirds of defects have no obvious explanation.

Cornell University's Dr Sam Flaxman and Professor Paul Sherman, who say their study is the first to produce compelling evidence of a beneficial effect, say their findings explain why many pregnant women avoid meats, caffeine and some strong tasting foods, and opt instead for a bland diet.

"Morning sickness is a complete misnomer. It is not a sickness in the pathological sense and we should change the name to wellness insurance," says Professor Sherman.

The protective theory for what the scientists call NVP - nausea and vomiting in pregnancy - is that it is a system the body has evolved for protecting mother and baby when the mother's immune system is naturally depressed so it will not reject the growing foetus. As a result, the mother's defences against food toxins and pathogens are also reduced.

"In the first three months when symptoms of NVP are most common, the cells of the embryo are starting to form structures and organs, like arms and legs, and eyes, as well as the central nervous system. It is the time when organ development is most susceptible to chemical disruption. At this critical stage, the foetus could be affected by chemicals in food," says Professor Sherman.

The researchers do not dispute the effects of maternal hormonal changes, but suggest that may simply be the mechanism by which nature triggers the protective sickness or aversion to food. Why, they ask, are the hormones producing sickness in pregnancy when they can in other circumstances, like the menopause, cause headaches.

Professor Sherman and his team found that the foods women most commonly avoided or which triggered nausea were meat, fish, eggs, and strong-tasting vegetables like broccoli and Brussels sprouts. They also point out that in a number of traditional societies where meat and animal products are not commonly eaten, there is almost no incidence of morning sickness.

The scientists found that women who have morning sickness are significantly less likely to miscarry than women who do not. They also report that women who vomit are less likely to miscarry than those who have nausea without being sick.

But there is comfort for women who don't get morning sickness: "Our analysis of thousands of pregnancies shows that most women in Western societies bear healthy babies whether or not they experience morning sickness. The lack of NVP symptoms does not portend pregnancy failure," says Professor Sherman whose research is reported in this month's Quarterly Review of Biology.

The researchers warn that attempts to ease the symptoms of normal levels of morning sickness will probably not improve the outcome of the pregnancy, and could have the opposite effect if the protective mechanism is undermined.

Although the research is the most compelling evidence yet for the idea of morning sickness being protective, the theory itself has been gaining support for some time.

Dr Margie Profet, author of Protecting Your Baby-to-Be, argues that morning sickness is so onerous on the mother that it would have been evolved-out if it did not serve a useful purpose.

She says that one of the reasons why many pregnant women avoid bitter foods, for example, is because they have more toxins than other food. Quitting coffee drinking, she points out, is also often the first dietary change a pregnant woman makes. Coffee has up to 1,000 different toxins.

The Cornell research is likely to prove controversial, but Professor Drife, one of Britain's leading specialists, says it needs to be considered. "It is interesting and it behoves us to keep an open mind about new theories because history tells us that a lot of theories were rubbished before people realised there was something in them," he says.

"My predecessor at Leeds, for example, fought for many years to show that spina bifida had a link with low folic acid levels. People scoffed for a long time before he was found to have been absolutely right."

'The Queen Charlotte's Hospital Guide to Pregnancy and Birth' by Adriana Hunter is published by Vermilion. 'Protecting Your Baby-to-Be' by Dr Margie Profet is published by Addison Wesley