Many pregnant women (about 70% to 85%) experience a period of nausea and vomiting. In the vast majority of cases, it's unpleasant but not dangerous.
About 1% of expectant mothers will vomit so severely that they lose weight and become dehydrated, requiring hospital treatment. This severe form of morning sickness is called hyperemesis gravidarum. Although this condition could be life-threatening if left untreated, it can be remedied by treatment during a brief hospital stay.
When a woman has morning sickness, it can be severe enough that it affects usual daily activities (e.g., working, caring for children). Although it's called "morning sickness," the nausea and vomiting may occur at any time of day.
While the exact causes of morning sickness aren't known, it's probably linked to pregnancy hormones. Women who suffer morning sickness tend to have higher levels of these hormones than those who don't.
It's possible that nausea and vomiting during pregnancy is an ancient mechanism for protecting the fetus from poisons. With the enormously rich and varied diet we eat today compared to our ancestors, it's no surprise that such a protective system could become a bit confused and start rejecting all sorts of healthy food.
One piece of evidence for this idea can be found in research showing that women who suffer from vomiting during pregnancy actually have lower rates of miscarriage than women who don't. Research shows higher levels of two hormones (thyroxine and human chorionic gonadotropin) in women with morning sickness. Women who suffer nausea from the birth control pill are more likely to develop morning sickness during pregnancy.