A 2016 study published in JAMA Internal Medicine found that among pregnant women with a history of miscarriage, those who experienced nausea and vomiting were 50-75% less likely to miscarry. A 2009 study published in The Journal of Pediatrics also suggests that children of mothers with morning sickness score higher on cognitive tests, including performance IQ and verbal fluency.
Nobody is totally sure why people have morning sickness during pregnancy, but the fact that it is associated with positive outcomes lends credence to the idea that it serves an adaptive function. Neurobiology professor Paul Sherman told the Cornell Chronicle that morning sickness may serve to discourage women from eating foods that could be harmful to their baby. This is supported by data that show morning sickness is more prevalent in societies that consume more alcohol and meat.
On the other hand, for the 3% of people with extreme morning sickness known as hyperemesis gravidarum, any benefits of nausea and vomiting are outweighed by the costs to the mother’s health (via the Cleveland Clinic). Hyperemesis gravidarum can lead to dehydration as well as deficiency in electrolytes and nutrition (per BMC Pregnancy and Childbirth). For this reason, the Cleveland Clinic recommends calling your doctor if you lose weight, vomit three or more times a day, or show signs of severe dehydration. Other symptoms that warrant a call to the doctor include dizziness, fainting, extreme tiredness, and a fast heartbeat.