The Facts
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.