Morning sickness (medically known as nausea and vomiting of pregnancy) is a term used to refer to nausea and often vomiting that typically occurs in early pregnancy. Morning sickness is very common and affects up to 75% of pregnant women at some time. It can be accompanied by other symptoms of early pregnancy, such as fatigue or tiredness. Despite the name, morning sickness or nausea can strike at any time during the day or night. Nausea of pregnancy usually starts about the sixth week of pregnancy, but some women may experience it earlier. Hyperemesis gravidarum is a term that refers to an unusually severe form of nausea and vomiting during pregnancy that often requires hospitalization.
Eating certain foods or experiencing certain smells may bring on nausea and vomiting of pregnancy. Being pregnant with twins or multiples is a risk factor for having morning sickness, possibly due to higher hormone levels. Other risk factors for morning sickness include a family or personal history of morning sickness, a history of migraine headaches, and a history of motion sickness. Carrying a girl may slightly increase the risk.
Causes of morning sickness
The exact cause of morning sickness is not understood, but it is believed to be due to the hormonal changes that occur in early pregnancy. For example, early in pregnancy, levels of the hormones estrogen and human chorionic gonadotropin (hCG) rise. There is also an enhanced sense of smell and sensitivity to odors in early pregnancy, which might contribute to nausea. It is possible that some women have a so-called sensitive stomach, meaning they are more sensitive to the hormonal changes of pregnancy than others. While there is no evidence to prove that this is true, some researchers have proposed that morning sickness might result from an abnormal response to stress early in pregnancy.
Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.
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