As irritating as morning sickness may be for pregnant women, it may protect embryos.
Doctors have long known that morning sickness — the nausea and vomiting usually experienced in early pregnancy — is actually a good sign of a healthy pregnancy, despite the discomfort it brings.
However, scientists have debated whether morning sickness actually helps pregnancies succeed.
It could just be an annoying byproduct of a healthy pregnancy, as pregnant women and their embryos carry out a tug of war over the body's resources.
When and why
To see which explanation might be right, scientists analyzed medical research to see when morning sickness does and does not occur.
If morning sickness was just the byproduct of a healthy pregnancy, then it should accompany all healthy pregnancies.
"But it doesn't," said researcher Samuel Flaxman, an evolutionary biologist at the University of Colorado at Boulder.
Although two-thirds of pregnant women do experience morning sickness, the rest often carry their pregnancies to term.
Also, morning sickness does not seem to occur in other mammals, only humans, the researchers noted.
If morning sickness was the byproduct of conflict between mother and embryo or fetus, one might expect other mammals to have it too.
Instead, morning sickness is usually triggered in specific circumstances — in response to:
— the sight, smell, or taste of meats and strong-tasting vegetables, which were historically likely to contain foodborne microbes or birth-defect-inducing chemicals;
— alcohol and cigarette smoke.
This all suggests morning sickness serves a useful function, evolving to protect mothers and embryos from things that may be dangerous, the researchers figure.