The etiology of nausea and vomiting in pregnancy (NVP) remains largely unknown. Research and theories have focused around 3 areas: psychological predisposition, evolutionary adaptation, and hormone response. A review of studies looking at the possible psychological etiology of NVP found little evidence to support the theory that NVP is a conversion disorder. This review, however, did note that there is evidence that the psychological response may be conditioned.  A second hypothesis is that NVP represents an evolutionary response to protect both the mother and the fetus from potentially harmful substances.  Most evidence supports the theory that elevated hormone levels are responsible for NVP. Progesterone has been shown to decrease gastric motility and cause nausea and vomiting in nonpregnant women.  Some studies have shown that NVP is associated with elevated human chorionic gonadotropin (hCG) levels.  Other studies have shown that NVP is more common in women with high estrogen levels and less common in those with lower estrogen levels.   Finally, a recent systematic review demonstrated an association between the presence of Helicobacter pylori and hyperemesis gravidarum. 
The underlying pathophysiology of nausea and vomiting in pregnancy is unclear, although it is understood that decreased gastric motility may play a role.