The causes of hyperemesis are likely varied, but a high level of the pregnancy hormone HCG is likely partly to blame. Elevated levels of estrogen can also have the same effect.
Hyperemesis gravidarum, or extreme nausea and vomiting during pregnancy, can affect up to two percent of pregnancies. Risk factors for hyperemesis include a history of having it in a prior pregnancy or a family history of hyperemesis, carrying twins or triplets, or carrying a female fetus. There is actually some data that shows that women with hyperemesis have a lower miscarriage rate, which is thought to be due to the robust placenta and elevated pregnancy hormone levels.
Reviewed by Dr. Jen Lincoln, December 2018