Hyperemesis gravidarum (HG) is the most severe form of nausea and vomiting during pregnancy (NVP). It is diagnosed clinically by its typical clinical picture and exclusion of other causes of nausea and vomiting in the pregnant woman. Nausea and vomiting are a common experience affecting 50% to 90% of all women. It is the most common indication for hospitalization during the first half of pregnancy. HG occurs in only about 0.5 to 2% of cases but when it occurs, it may be associated with morbidity of both mother and fetus. There are many risk factors related to HG, but we are still in need of adequate clinical research methodology and design for reporting these associations. In this article, we will discuss them and review epidemiological studies and statistics that support them. HG is found to be common in young aged mothers. There is a debate between studies regarding ethnicity as a risk factor. Low socio-economic level is found to be associated with HG. Dietary factors are considered in Indians who are mostly vegan and Asians who are mostly lactose intolerant. Deficiencies of trace elements are associated with more progression of the disease. Women affected by NVP are mostly nonsmoker. Molar pregnancy, multiple pregnancies, fetus with Down syndrome and primi-gravidity are common obstetric risk factors. Studies theorize relation between hyperthyroidism and HG based on structural similarity between hCG and TSH but more studies are needed to highlight this relation. Mood and anxiety disorders are associated with the disease but causal relation has not been established yet. GERD, peptic ulcer disease and specifically H. pylori infection are among common risk factors. Asthma is associated with prolonged duration of hospitalization. Strong positive family history of HG in patients highlights genetic predisposition of this disease.