ABSTRACTObjectives: To assess the hydration status of women presenting to an ED with hyperemesis gravidarum and to determine whether clinically relevant changes in orthostatic vital signs occur.Methods: A convenience sample of 23 pregnant women who had hyperemesis gravidarum, with each patient serving as her own control. The study took place in the ED observation unit of an urban teaching hospital. Women who had pregnancies of ≤16 weeks' gestation who had been vomiting for at least 24 hours were included. Supine and standing pulse rates and blood pressures (BPs) were measured sequentially after 5 minutes in each position. Patient weight and urine specific gravity (SG) also were recorded. After 6 L of lactated Ringer's solution was infused over a 12‐hour period, the same measurements were repeated. Pre‐ and posthydration changes were analyzed using the paired t‐test.Results: The mean treatment weight gain as a percentage of the total body weight was 5.6% ± 2.2% (mean ± SD). The urine SG decreased from 1.027 ± 0.004 to 1.008 ± 0.003 (p < 0.001). The mean change in systolic BP upon assuming the standing position was ‐8.3 ± 12.7 mm Hg before hydration vs 2.9 ± 7.8 mm Hg after hydration (p < 0.001). The corresponding change in mean diastolic BP was 3.7 ± 10.9 mm Hg before hydration vs 8.6 ± 10.9 mm Hg after hydration (p = 0.12). The mean change in pulse rate upon standing was 26.8 ± 14.5 beats/min before hydration vs 14.5 ± 10.1 beats/min after hydration (p = 0.002).Conclusions: Women who present to the ED with hyperemesis gravidarum are significantly dehydrated and experience measurable improvement in postural pulse rate and systolic BP changes with rehydration. However, the presenting orthostatic changes lack sufficient sensitivity to be effectively used as quantitative screening tests for dehydration.