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Morning sickness
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==Pathophysiology== ===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 = 639β644 | 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 = 1544β1550 | date = October 2010 | pmid = 20942670 | doi = 10.1056/NEJMcp1003896 }}</ref> ===Defense mechanism=== Morning sickness may be an [[natural selection|evolved trait]] that protects the fetus against [[toxin]]s ingested by the mother. Biologist Margie Profet believes that nausea and food aversions during pregnancy evolved to impose dietary restrictions on the mother in the early weeks of pregnancy, when the mother and the embryo are most immunologically vulnerable, to minimize fetal exposure to toxins such as mutagens and teratogens.<ref>{{Cite book |title=Why We Get Sick: The New Science of Darwinian Medicine. |vauthors=Holt RD, Nesse RM, Williams GC |date=April 1996 |isbn=978-0679746744 |pages=983|publisher=Knopf Doubleday Publishing }}</ref> By reducing exposure to such chemicals, morning sickness reduces impairments on normal embryonic development and increases the reproductive success of the mother and survival success of both the mother and her offspring. Evidence in support of this theory includes:<ref name="Nesse">{{cite book | vauthors = Nesse RM, Williams GC |author-link2=George C. Williams (biologist) |title= Why We Get Sick |year=1996 |edition=1st |publisher= Vintage Books|location= New York |pages=290 }}</ref><ref name = "Pepper_2006" /> * Morning sickness is very common among pregnant women, which argues in favor of its being a functional adaptation and against the idea that it is a [[pathology]]. * Fetal vulnerability to toxins peaks at around 3 months, which is also the time of peak susceptibility to morning sickness. * There is a good correlation between toxin concentrations in foods, and the tastes and odors that cause revulsion. Women who have ''no'' morning sickness are more likely to [[miscarriage|miscarry]].<ref>{{cite journal | vauthors = Chan RL, Olshan AF, Savitz DA, Herring AH, Daniels JL, Peterson HB, Martin SL | title = Severity and duration of nausea and vomiting symptoms in pregnancy and spontaneous abortion | journal = Human Reproduction | volume = 25 | issue = 11 | pages = 2907β2912 | date = November 2010 | pmid = 20861299 | pmc = 3140259 | doi = 10.1093/humrep/deq260 | author4-link = Amy H. Herring }}</ref><ref>{{Cite web |last=Collins |first=Dr Francis |date=2016-10-04 |title=Morning Sickness Associated with Lower Miscarriage Risk |url=https://directorsblog.nih.gov/2016/10/04/nausea-in-pregnancy-is-associated-with-lower-miscarriage-risk/ |access-date=2023-06-25 |website=NIH Director's Blog |language=en-US}}</ref> This may be because such women are more likely to ingest substances that are harmful to the fetus.<ref>{{cite journal | vauthors = Sherman PW, Flaxman SM | title = Nausea and vomiting of pregnancy in an evolutionary perspective | journal = American Journal of Obstetrics and Gynecology | volume = 186 | issue = 5 Suppl Understanding | pages = S190βS197 | date = May 2002 | pmid = 12011885 | doi = 10.1067/mob.2002.122593 | citeseerx = 10.1.1.611.7889 }}</ref> In addition to protecting the fetus, morning sickness may also protect the mother. A pregnant woman's [[immune system]] is suppressed during pregnancy, presumably to reduce the chances of [[transplant rejection|rejecting]] tissues of her own offspring.<ref>{{cite journal | vauthors = Haig D | title = Genetic conflicts in human pregnancy | journal = The Quarterly Review of Biology | volume = 68 | issue = 4 | pages = 495β532 | date = December 1993 | pmid = 8115596 | doi = 10.1086/418300 | s2cid = 38641716 | url = http://nrs.harvard.edu/urn-3:HUL.InstRepos:3153297 | author-link = David Haig (biologist) }}</ref> Because of this, animal products containing [[parasite]]s and harmful bacteria can be especially dangerous to pregnant women. There is evidence that morning sickness is often triggered by animal products including meat and fish.<ref name = "Flaxman_2000">{{cite journal | vauthors = Flaxman SM, Sherman PW | title = Morning sickness: a mechanism for protecting mother and embryo | journal = The Quarterly Review of Biology | volume = 75 | issue = 2 | pages = 113β148 | date = June 2000 | pmid = 10858967 | doi = 10.1086/393377 | s2cid = 28668687 }}</ref> If morning sickness is a defense mechanism against the ingestion of toxins, the prescribing of [[anti-nausea drugs|anti-nausea medication]] to pregnant women may have the undesired [[side effect]] of causing birth defects or miscarriages by encouraging harmful dietary choices.<ref name="Nesse"/> Also, morning sickness is a defense mechanism because when analyzing embryonic growth, several critical periods are identified in which there is mass proliferation and cell division resulting in the development of the heart and central nervous system that are very sensitive. In that period, the fetus is most at risk from damage to toxins and mutagens. These developments occur through week 6-18 which is in the same time frame in which the most nausea and vomiting of pregnancy (NVP) occurs. This relationship between the time at which the embryo is most susceptible to toxins lines up exactly with when the most severe NVP symptoms are seen, suggesting that this NVP is an evolutionary response developed in the mother, to indicate the sensitivity of the fetus hence making her wary to her health and in turn protecting the fetus.<ref name = "Flaxman_2000" />
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