Hyperemesis gravidarum is extreme, persistent nausea and vomiting occuring during pregnancy, which may lead to dehydration.
Nearly all women experience some degree of "morning sickness" during their pregnancy (especially during the first trimester). The cause is believed to be the surge of a pregnancy hormone called HCG (human chorionic gonadotropin)secreted by the fetus. Increased estrogen levels have also been implicated.
However, too much vomiting can interfere with the weight gain needed to sustain the pregnancy and cause dehydration, which can be harmful to both mother and child. Extreme nausea and vomiting during pregnancy may indicate a multiple pregnancy (twins or more) or, extremely rarely, a hydatidiform mole or thyroid disorder.