Morning sickness, also called nausea gravidarum, nausea, vomiting of pregnancy (emesis gravidarum or NVP), or pregnancy sickness is a pregnancy discomfort that affects more than half of all pregnant women. Sometimes symptoms are present in the early hours of the morning and reduce as the day progresses. However, in spite of its common name, it can occur at any time of the day. For most women it may stop around the 12th week of pregnancy.
An increase in salivation during the first trimester, that is often bitter tasting (Ptyalism), is then ingested during the mother's sleep. This can upset the stomach enough to cause the morning nausea.
An increase in the circulating level of the hormoneestrogen. 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.
Low blood sugar (hypoglycemia) due to the placenta's draining energy from the mother, though studies have not confirmed this except for in Type I diabetic expectant mothers.
An increase in human chorionic gonadotropin. It is probably not the human chorionic gonadotropin itself that causes the nausea. More likely, it is the human chorionic gonadotropin stimulating the maternal ovaries to secrete estrogen, which in turn causes the nausea.
An increase in sensitivity to odors, which overstimulates normal nausea triggers.
An increase in bilirubin levels due to increased liver enzymes.
Morning sickness is understood as an evolved trait that protects the fetus against toxins ingested by the mother. Many plants contain chemical toxins that serve as a deterrent to being eaten. Adult humans, like other animals, have defenses against plant toxins, including extensive arrays of detoxification enzymes manufactured by the liver and the surface tissues of various other organs. In the fetus, these defenses are not yet fully developed, and even small doses of plant toxins that have negligible effects on the adult can be harmful or lethal to the embryo. Pregnancy sickness causes women to experience nausea when exposed to the smell or taste of foods that are likely to contain toxins injurious to the fetus, even though they may be harmless to her.
There is considerable evidence in support of this theory, including:
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 miscarry. This may be because such women are more likely to ingest substances that are harmful to the fetus.
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 rejecting tissues of her own offspring. Because of this, animal products containing parasites 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.
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. On the other hand, many domestic vegetables have been purposely bred to have lower levels of toxins than in the distant past, and so the level of threat to the embryo may not be as high as it was when the defense mechanism first evolved.
There is unclear evidence about the effectiveness of home treatments for morning sickness. 
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^Tiran, Denise (Feb 2012). "Ginger to reduce nausea and vomiting during pregnancy: Evidence of effectiveness is not the same as proof of safety". Complementary Therapies in Clinical Practice18 (1): 22. doi:10.1016/j.ctcp.2011.08.007. ISSN1744-3881.