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Possible biological causes behind pregnant women experiencing morning sickness:

 * There is an increase in the circulating level of estrogen.[1] However, there is no consistent evidence of differences in estrogen levels and bilirubin levels between women that experience sickness and those who do not.[2]
 * There may be lower blood sugar due to the body placing the fetus’ nutrient needs over the mother’s. However, this theory has only been confirmed in Type 1 diabetic patients. [3,4]
 * An increase in progesterone levels relaxes the muscles in the uterus, which prevents early childbirth, but may also relax the stomach and intestinal muscles, which leads to excess stomach acids and gastroesophageal reflux disease (GERD).
 * An increase in human chorionic gonadotropin leads to a higher stimulation of the maternal ovaries. This stimulation causes the ovaries to secrete more estrogen, which in turn causes nausea.[5]
 * An increase in sensitivity to odors, which overstimulates normal nausea triggers.
 * Excess salivation that is often bitter-tasting (ptyalism) during the first trimester, which is ingested during the mother’s sleep.[27]

Protection Hypothesis:
Morning sickness may be an evolved trait and defense mechanism that protects the baby against toxins and/or parasites ingested by the mother. This is known as the “prophylaxis”, or “maternal and embryonic protection”, hypothesis and is the most commonly accepted theory.[6, 9, 11, 24] Evidence in support of this theory include: There is a key correlation between toxin concentrations in foods and the tastes and odors that commonly cause revulsion. Foods that may trigger morning sickness include:
 * Morning sickness is very common among pregnant women, suggesting a functional adaptation rather than pathological.
 * Fetal vulnerability to toxins peaks at around 3 months, which is also the time of peak susceptibility to morning sickness.
 * Pregnant women experience a temporarily weakened immune response as the embryo burrows into the uterine wall. The suppression of the immune response leaves the mother vulnerable to harmful substances. Thus, morning sickness may also be a protective measure for pregnant women. [25]
 * Women who experience morning sickness are less likely to suffer a miscarriage.[8]
 * Animal products: animal products that have strong odors, such as meat and fish, are typically more likely to contain pathogens or parasites than other foods.[11]
 * Plants with phytochemicals: includes strong or bitter-tasting vegetables, such as broccoli, cabbage, or brussel sprouts. These vegetables are likely to contain secondary compounds. Since plants use such chemicals to defend themselves from herbivores, the compounds can harm individuals that ingest them.[9]

This evidence may be correlated with the increased toxicity of women’s diets in the given areas.[9] Women from certain cultures may experience less morning sickness than others. These societies were found to have diets based on corn, rice, tubers, or other plants. These plants contain little or no phytochemicals, which may explain low levels of morning sickness. Morning sickness is almost non-existent in societies with rare animal-product consumption.[7]

If morning sickness is a defense mechanism against the ingestion of toxins, the prescribing of anti-nausea medication to pregnant women may have the undesired side effect of causing birth defects or miscarriages by encouraging harmful dietary choices.[6, 11]

“By-product” hypothesis:

 * This suggests that morning sickness serves as a nonfunctional by-product due to conflict between the mother and fetus for resources. This hypothesis has not been subjected to as much scrutiny and analysis as the prophylaxis theory due to the lack of evidence supporting this hypothesis.[22, 23]

“Genetic Conflict” hypothesis:

 * This theory proposes that morning sickness is not directly adaptive, but is instead a result of conflict between the mother and fetus. The theory proposes that nausea and vomiting are caused when the fetus releases human chorionic gonadotropin (hCG). Since low quality embryos or those with genetic defects will produce lower levels of hCG, the theory suggests that morning sickness may be sign of a healthy fetus.[10]

“Maternal-Fetal Conflict” hypothesis:

 * This hypothesis suggests that morning sickness may arise from genetic conflicts between the mother and fetus. [26] More specifically, proponents of this theory argue that fetal genes are selected to increase the transfer of nutrients to the fetus, whereas maternal genes are selected to limit such transfers in order to protect the mother. This can go hand-in-hand with placental hormones that are hypothesized to manipulate the maternal physiology for fetal benefit. The hypothesis suggests that these hormonal mechanisms may play a role in contributing to morning sickness during pregnancy. [10]

References [to add to those already on the Wikipedia page]:

22. Flaxman, Samuel M., and Paul W. Sherman. "Morning Sickness: Adaptive Cause or Nonadaptive Consequence of Embryo Viability?" The American Naturalist 172.1 (2008): 54-62. Web. 13 Mar. 2016.

23. Forbes, Scott. "Pregnancy Sickness and Embryo Quality." Trends in Ecology & Evolution 17.3 (2002): 115-20. Web. 14 Mar. 2016.

24. Profet, Margie (1988), "The Evolution of Pregnancy Sickness as Protection to the Embryo Against Pleistocene Teratogens", Evolutionary Theory 8, pp. 177–190.

25. Sydney B., Cooper M.B., M.R.C.O.G. “Ptyalism Complicating Pregnancy” BJOG An International Journal of Obstetrics and Gynaecology 63.4 (1956): 582-588. Web. 14 Mar. 2016

26. Zimmer, Carl. “Silent Struggle: A New Theory of Pregnancy.” New York Times 14 March 2006. Web.