Morning Sickness Causes

Morning sickness, which nausea and vomiting affect about two out of every three pregnant women.

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Protective role

One hypothesis that has been proposed to explain morning sickness is that the nausea and vomiting can protect the fetus from noxious chemicals, especially those which are teratogenic or capable of causing abortion. More specifically, morning sickness can often cause the physical expulsion of certain strongly flavored foods, caffeine-containing drinks, and alcohol.

This hypothesis is supported by the fact that fetal organs are formed between the 6th and 18th weeks of pregnancy; therefore, the fetus is most vulnerable to chemical teratogens during this time period. Unsurprisingly, this is also the period when maximum morning sickness is found to occur. Again, women with morning sickness have a lower rate of miscarriage as compared to those without it.

Another curious finding is that morning sickness is more pronounced in women coming from communities in which animal food products are a necessary part of their daily diet. In these women, their highest reported food aversions include meats, eggs, chicken and other poultry, as well as fish.

There have been seven societies that have not reported a history of morning sickness, most of which primarily consumed plant-based diets, particularly those that included corn. The possible explanation may be that the body resists animal meats, which may contain bacteria and parasites that are capable of causing dangerous infections, especially before the onset of refrigeration. As pregnancy is an immunosuppressed state, the risk of infection is higher.

Another non-validated hypothesis is that morning sickness arises from conflicting genetic matter or hormones in the maternal and fetal makeup. The morning sickness could therefore indicate to other potential partners that the woman is pregnant, thus reducing the risks of sexual behavior and increasing the chances of her getting help to ensure the protection of a healthy offspring.

Combination of causes

The conclusion today is that despite many theories, the etiology of morning sickness is due to a combination of hormones, immunologic factors, anatomical factors, and even mental stress. Some risk factors have been proposed, including:

  • Younger age of mothers
  • Increased placental weight, as can occur in multiple gestations or macrosomia
  • Genetic tendency
  • History of motion sickness/migraines/nausea following the use of oral contraceptive pills containing estrogen
  • Prior history of severe morning sickness
  • Multiparity or already having given birth to one or more children (though, in contrast, one of the largest studies has shown primiparity to be a risk factor)
  • Female fetal gender increases the risk by about 50%
  • Infection with Helicobacter pylori (H. pylori)
  • Women with less than 12 years of formal education
  • Obese or underweight women, though this remains to be established
  • Ethnic factors, such as reduced duration of morning sickness in Hispanic and black women
  • High pre-pregnancy intake of total fat

Smoking and alcohol consumption reduces the chances of morning sickness but is associated with a higher risk to the fetus. Pre- and early pregnancy supplementation with vitamins reduces the risk of experiencing morning sickness. Meals rich in protein can also decrease the symptoms of morning sickness.

What Causes Morning Sickness?

Hyperemesis gravidarum

Hyperemesis gravidarum (HG) is the extreme end of the morning sickness spectrum, and risk factors for this include personal or family history of HG, multiple gestations, trophoblastic disease, and fetal chromosomal abnormalities. Married women or those with stable partners have a higher risk as compared to those who have conceived for the first time.


The common factor between all of the proposed causes is thought to be human chorionic gonadotropin (hCG), as this substance reaches its peak in the period associated with the highest incidence of morning sickness, namely, 12-14 weeks of gestation, and because its levels are higher in such women compared to those without such symptoms.

Moreover, the higher the hCG levels, the worse the nausea and vomiting generally are. This finding has not been universally accepted, however. Again, different hCG isoforms exist, with different effects on the receptors for luteinizing hormone (LH) and thyroid-stimulating hormone (TSH). These may vary in different women, accounting for the variation in morning sickness between individuals.

Estrogen and progesterone are also implicated, and these are high in women with low parity or increased body mass. Estrogen slows gastric transit time and emptying via a nitric oxide-mediated mechanism, which may subsequently contribute to the phenomenon of morning sickness. Progesterone may also do the same thing. However, it is not proven that gastric motility does indeed slows down during pregnancy, though intestinal motility is affected.

Other cytokines such as prostaglandin E2 (PGE2), interleukin 1β (IL-1β), and tumor necrosis factor α (TNFα) may also be involved; however, the evidence is inconclusive. Increased immunologic activation has been thought to take place, as shown by high levels of T-helper cell activation, IL-4, TNF-α, IL-6, and other molecules, but this may be a compensatory reaction to the normal immunosuppression resulting from starvation, which may occur in HG.

H. pylori is a possible risk factor for morning sickness, and higher levels of infection are possibly associated with higher severity. Its role may be to enhance the hormone and neural dysfunction of the stomach, worsening nausea and vomiting in women infected by this infection.

Most women, however, are asymptomatic. Some studies do suggest that eradication of H. pylori does reduce morning sickness and should be done in cases when the symptoms resist all forms of conventional treatment. Otherwise, treatment should be given after lactation is completed.

Finally, it is postulated that morning sickness may be a physical manifestation of mental conflict over the presence of the baby, as it is more likely to happen with unplanned or unwanted pregnancies. Other mental disorders associated with HG include neurosis, depression over poverty or family conflicts, as well as a lack of acceptance of one’s femininity and/or of the pregnancy. However, much more evidence is needed, as the disorder of the psyche could be the result and not the cause of the vomiting.

Further Reading

Last Updated: Apr 27, 2021

Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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