Hyperemesis gravidarum is a severe and intractable form of nausea and vomiting in pregnancy, affecting 0.8-2.3% of pregnant women. [2, 3] It is a diagnosis of exclusion and may result in weight loss; nutritional deficiencies; and abnormalities in fluids, electrolyte levels, and acid-base balance. The peak incidence is at 8-12 weeks of pregnancy, and symptoms usually resolve by week 20 in all but 10% of patients. Uncomplicated nausea and vomiting of pregnancy is generally associated with a lower rate of miscarriage, but hyperemesis gravidarum may affect the health and well-being of both the pregnant woman and the fetus.
The etiology of nausea and vomiting of pregnancy is unknown. Many have postulated that nausea and vomiting are protective in pregnancy to reduce exposures to potentially teratogenic materials. Some theories hold that elevated human chorionic gonadotropin (hCG) or estradiol levels could be causative, due to correlations in numerous studies between levels and symptoms, but this has not been demonstrated conclusively. Psychological theories of the etiology are falling out of favor, and the American College of Obstetrics and Gynecology warns that attributing vomiting to psychological disorders has likely impeded progress in understanding the true etiology of hyperemesis gravidarum. 
The cause of severe nausea and vomiting in pregnancy has not been identified. Hyperemesis may have a genetic component, as sisters and daughters of women with hyperemesis have a higher incidence.
Hyperemesis is also associated with hyperemesis in prior pregnancy, female gestation, multiple gestation, triploidy, trisomy 21, current or prior molar pregnancy, and hydrops fetalis.
Women with history of motion sickness, migraine headaches, psychiatric illness, pregestational diabetes, being underweight pregestation,  hyperthyroidism, pyridoxine deficiency, and gastrointestinal disorders are also at an increased risk.
Some studies have suggested that Helicobacter pylori infection may play a role in hyperemesis,  but the data are inconclusive.
Cigarette smoking and maternal age older than 30 years appear to be protective.
United States statistics
Hyperemesis gravidarum occurs in 0.5-2% of pregnancies, with the variation in incidence arising from different diagnostic criteria and ethnic variations.  Studies have found an admission rate of 0.8% for hyperemesis gravidarum  and an average of 1.3 hospital admissions per hyperemesis patient, with an average hospital stay of 2.6-4 days.
Race- and age-related demographics
Hyperemesis patients are more likely to be nonwhite.
Patients younger than 30 years are more likely to experience hyperemesis.
One study has demonstrated that adverse fetal outcomes are mostly limited to poor maternal weight gain.  Women who gained less than 7 kg in pregnancy were more likely to have fetal complications, but those with hyperemesis and greater than 7 kg weight gain had no increased risk. This research indicates that treating hyperemesis gravidarum such that the patient is able to gain weight portends a better prognosis.
With mild-to-moderate vomiting, the patient and the fetus are unlikely to experience any increased morbidity or mortality. Before the advent of intravenous hydration, hyperemesis was a major cause of maternal death. Currently, mortality is exceedingly rare, but maternal morbidities may include Wernicke encephalopathy from vitamin B-1 deficiency, Mallory-Weiss tears, esophageal rupture, pneumothorax, and acute tubular necrosis. Hyperemesis is the second leading cause of hospitalization in pregnancy, second only to preterm labor. Additionally, many women experience significant psychosocial morbidity, occasionally interfering with assumption of the maternal role and rarely leading to termination of the pregnancy.
Complications of vomiting rarely occur; however, Mallory-Weiss tears and esophageal perforations have been reported.
Women with hyperemesis and poor weight gain have lower average birth weights and are more likely to have a small for gestational age infant and may be at higher risk for preterm birth.
In severe cases, without thiamine supplementation, Wernicke encephalopathy may occur (ie, diplopia, nystagmus, disorientation, confusion, coma).
If treatment is unsuccessful, complications of prolonged dehydration and starvation may occur.