Editing Morning sickness (section)

===Hormone changes===
[[File:Morning sickness.svg|thumb|300px|Pathophysiology of vomiting in pregnancy]]
* An increase in the circulating level of the hormone [[estrogen]].<ref>{{cite journal | vauthors = Lagiou P, Tamimi R, Mucci LA, Trichopoulos D, Adami HO, Hsieh CC | title = Nausea and vomiting in pregnancy in relation to prolactin, estrogens, and progesterone: a prospective study | journal = Obstetrics and Gynecology | volume = 101 | issue = 4 | pages = 639–644 | date = April 2003 | pmid = 12681864 | doi = 10.1016/s0029-7844(02)02730-8 | s2cid = 13103469 }}</ref> However, there is no consistent evidence of differences in estrogen levels and levels of [[bilirubin]] between women that experience sickness and those that do not.<ref>{{cite web|url=http://www.nymetroparents.com/newarticle.cfm?colid=7114|title=Morning Sickness: Coping With The Worst|publisher=NY Metro Parents Magazine|access-date=2008-07-06| vauthors = Bauchner E, Marquez W |url-status=dead|archive-url=https://web.archive.org/web/20081204115500/http://www.nymetroparents.com/newarticle.cfm?colid=7114|archive-date=2008-12-04}}</ref> Related to increased [[estrogen]] levels, a similar form of nausea is also seen in some women who use [[hormonal contraception]] or [[Hormone replacement therapy (menopause)|hormone replacement therapy]], but does not predict future reactions to pregnancy.
* An increase in [[progesterone]] relaxes the [[muscle]]s in the [[uterus]], which prevents early [[childbirth]], but may also relax the [[stomach]] and [[intestine]]s, leading to excess [[stomach acid]]s and [[gastroesophageal reflux disease|gastroesophageal reflux disease (GERD)]].
* An increase in [[human chorionic gonadotropin]]. It is probably not the HCG itself that causes the nausea. More likely, it is the HCG stimulating the maternal ovaries to secrete estrogen, which in turn causes the nausea.<ref>{{cite journal | vauthors = Niebyl JR | title = Clinical practice. Nausea and vomiting in pregnancy | journal = The New England Journal of Medicine | volume = 363 | issue = 16 | pages = 1544–1550 | date = October 2010 | pmid = 20942670 | doi = 10.1056/NEJMcp1003896 }}</ref>