Hyperemesis gravidarum: Extreme, excessive, and persistent vomiting in early pregnancy that may lead to dehydration and malnutrition. It is usually associated with weight loss of more than 5%of the woman's prepregnancy weight. Hyperemesis gravidarum affects about 1 in every 300 pregnant women and is most common in young women, in first pregnancies, and in women carrying multiple fetuses. Hyperemesis gravidarum usually stops on its own by the 20th week of pregnancy. Treatment of mild hyperemesis gravidarum usually involves dietary measures, rest, and use of antacids. Very severe hyperemesis gravidarum may call for the use of intravenous fluids and nutrition.