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Morning sickness, also called nausea gravidarum, nausea, vomiting of pregnancy (emesis gravidarum or NVP), or pregnancy sickness is a condition that affects more than half of all pregnant women, as well as some women who use hormonal contraception or hormone replacement therapy. Usually, it is present in the early hours of the morning and reduces as the day progresses. The nausea can be mild or induce actual vomiting. In extreme cases, vomiting may be severe enough to cause dehydration, weight loss, alkalosis and hypokalemia. This extreme condition is known as hyperemesis gravidarum and occurs in about 1% of all pregnancies. Nausea and vomiting can be one of the first signs of pregnancy and usually begins around the 6th week of pregnancy (week 1 starting on the day the last period started). It can occur at any time of the day, and for most women it seems to stop around the 12th week of pregnancy.


Proximate causes of pregnancy sickness include:

Morning sickness as a defense mechanism

Morning sickness is currently 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 it 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 or to bear children with birth defects.

In addition to protecting the fetus, morning sickness may also protect the mother. Pregnant women's immune systems are suppressed during pregnancy, presumably to reduce the chances of rejecting tissues of their 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.

Other explanations

Many other non-scientific theories for morning sickness have been proposed in the past. Notably, according to psychologist Sigmund Freud, morning sickness is the result of the mother's loathing of her husband. The subconscious manifestation of this is a desire to abort the fetus through vomiting. In general, such theories are not accepted by modern scientists; Steven Pinker, in "How the Mind Works" goes further, ridiculing the idea as the "barf-up-your-baby theory".


Treatments for morning sickness typically aim to lessen the symptoms of nausea, rather than attacking the root cause(s) of the nausea. Treatments include:

  • Lemons, particularly the smelling of freshly cut lemons.
  • Avoiding an empty stomach.
  • Accommodating food cravings and aversions.
  • Eating five or six small meals per day, rather than three large ones.
  • Eating cabbage.
  • Trying the BRAT diet: bananas, rice, applesauce, toast and tea.
  • Ginger, in capsules, tea, ginger ale, or ginger snaps.
  • Eating dry crackers in the morning.
  • Drinking liquids 30 to 45 minutes after eating solid food.
  • If liquids are vomited, sucking ice cubes made from water or fruit juice.
  • Vitamin B6 (either pyridoxine or pyridoxamine), often taken in combination with the antihistamine doxylamine (Diclectin).

A doctor may prescribe anti-nausea medications if the expectant mother suffers from dehydration or malnutrition as a result of her morning sickness, a condition known as hyperemesis gravidarum. In the US, Zofran (ondansetron) is the usual drug of choice, though the high cost is prohibitive for some women; in the UK, older drugs with which there is a greater experience of use in pregnancy are preferred, with first choice being promethazine otherwise as second choice metoclopramide, or prochlorperazine.


Thalidomide was originally developed and prescribed as a cure for morning sickness in West Germanymarker, but its use was discontinued when the drug's teratogenic properties came to light. The United Statesmarker Food and Drug Administration never approved thalidomide for use as a cure for morning sickness.


  • Morning Sickness: A Comprehensive Guide to the Causes and Treatments, Nicky Wesson, Vermilion (1997), ISBN 009181538X
  • Morning Sickness - A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References, ICON Health Publications (2004), ISBN 0597840431


  1. American Pregnancy Association,
  2. Pinker, Steven (1997) How the Mind Works ISBN 978-0-140-24991-5, p.39

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