Morning sickness might have evolved to ensure pregnant women do not
digest too much unhealthy food, say scientists.
They found evidence that nausea and vomiting in pregnancy is associated with high
intake of sugar, alcohol, oils and meat.
In contrast, cereals were least likely to trigger sickness.
The University of Liverpool analysis of 56 previous studies in 21 countries
appears in the Royal Society's Biological Journal.
Morning sickness occurs in the first three months of up to 80% of pregnancies.
Until recently, it was thought to be simply an unfortunate by-product of dramatic
hormonal changes during early pregnancy.
However, research has suggested that morning sickness might have positive consequences
- such as a reduced risk of miscarriage.
The researchers believe it is possible that the pregnant human body may have
evolved an aversion to foods containing high levels of toxins, and that this
may have carried over into modern living.
They suggest that the body might reject meat because of the relatively high
risk that it might harbour disease-causing agents.
Conversely, the low level of plant toxins in cereals may make them particularly
unlikely to trigger nausea.
However, they say the body's rejection of sugars and oils is less easy to explain.
Lead researcher Dr Craig Roberts said: "While there may be no particular
harm in eating, say, meat, now that we have refrigeration and best before dates,
our bodies may be pre-programmed by evolution to avoid these particular foodstuffs
in the first trimester.
"It may be that the nausea women feel towards certain foodstuffs could
be helpful, and that although it is inconvenient and miserable, their nausea
could be nature's way of avoiding problems in pregnancy for both mother and
Dr Maggie Blott, a consultant obstetrician at London's King's College Hospital,
said: "I can understand why mother nature might do this.
"Morning sickness is always worst in the first three months, which is
when the most important part of a foetus's development is happening."
Dr Blott said while it was sensible for a woman to eat healthily in pregnancy,
it was more important to avoid alcohol, smoking or drugs.
It was also important to take folic acid supplements, and to avoid high levels
of vitamin A.