The theory is based on the idea that sickness in the early stages of pregnancy is a response to very small quantities of plant or bacterial toxins that would be harmful - or even fatal - to the foetus at a crucial stage in its development.
Dr Margie Profet, a theoretical biologist in the Division of Biochemistry and Molecular Biology, Univerisity of California at Berkeley, says that the sickness is experienced equally by women of hunter-gatherer tribes and women in the developed world.
It usually starts at two weeks when the cells of the embryo begin to divide, peaks at eight weeks and has gone by the 14th week. 'It is when the foetus is extremely vulnerable', she says.
Dr Profet's ideas, reported in the American journal Science, go against the orthodox explanation - that sickness in pregnancy is caused by changes in hormone levels which the woman takes time adjusting to.
She claims the surge in oestrogen sensitises a primitive part of the brain, making it very responsive to the presence of toxins from diet. This produces the desired effect of nausea and vomiting, and aversion to the offending substance.
'What struck me was that while nausea and vomiting is always mentioned in the literature on pregnancy sickness, food aversion is not. Yet it is a very common experience of women everywhere'.
The defence mechanism, she said, was not something left-over from primitive woman but was active today against coffee, tea, spicy food and bitter vegetables.Reuse content