Why is it called 'morning sickness' if it can happen any time of day?

Nausea and vomiting are extremely common in the early stages of pregnancy, and they've earned the widely used nickname "morning sickness." 

But why exactly have we come to call these tummy-turning symptoms morning sickness? 

It turns out that, while pregnancy-related nausea and vomiting can happen at any time of day, the symptoms often flare just after waking up. Scientists don't know exactly why, but in general, studies suggest that hormonal changes in early pregnancy may drive morning sickness, according to a 2016 review published in the International Journal of Reproduction, Contraception, Obstetrics and Gynecology. 

A person's levels of reproductive hormones surge in the first trimester. One hormone, called human chorionic gonadotropin (hCG), may be directly responsible for the queasiness tied to morning sickness, Dr. Clara Paik, a professor and vice-chair of the Department of Obstetrics and Gynecology at UC Davis Health, told Live Science by email. 

Related: Am I pregnant? 12 early signs of pregnancy 

"We think that this hormone is associated with pregnancy sickness because it is usually during the first trimester when this hormone is produced that women feel the most nausea," she said. However, the exact mechanism of how hCG drives the queasiness is not clear. 

According to the 2016 review, some researchers speculate that hCG may induce nausea by stimulating the secretion of fluids in the digestive tract. The hormone has also been linked to a rapid increase in the level of thyroxines — thyroid-made hormones that are involved in regulating digestion and help control how quickly food moves through the digestive tract.

The hormones estrogen and progesterone may also fuel symptoms of pregnancy sickness, Dr. Adiele Hoffman, a general practitioner and medical advisor at the period-tracking app Flo Health, told Live Science. 

"These hormones relax muscles in your stomach and intestines, slowing down digestion, meaning your last meal might hang around for a little longer and sometimes, unfortunately, reappear," she said. 

Hormones may also explain why some people get severe morning sickness while others get none at all. "Everyone's level of these hormones varies, as does how their body responds to hormonal changes," Hoffman noted. 

People's genetics may also influence how sick they feel during pregnancy sickness, according to a 2019 review published in the journal Nature Reviews Disease Primers.

In addition to hormones and genetics, hypoglycemia, or low blood sugar, can worsen nausea, and in general, our blood sugar levels are lowest when we wake up. Pregnancy can lead to even bigger drops in blood sugar than one would typically experience overnight, due to the energetic needs of the growing fetus, according to the 2016 review. 

When hypoglycemia couples with the relaxation of muscles in the stomach and intestines, nausea can be the unfortunate consequence, the review authors wrote. Moreover, symptoms of pregnancy sickness may be more pronounced in the morning both due to a person having an empty stomach and experiencing a temporary drop in blood pressure when they get out of bed, Paik said.  

Symptoms of morning sickness can also increase with stress, anxiety and fatigue, Paik noted. In both people who are pregnant and those that aren't, negative emotions can fuel digestive discomfort due to the close link between the nervous and digestive systems.  

People who are more likely to feel nauseated, in general — such as those who are prone to motion sickness, feel nauseous during migraines or get nausea as a side effect of estrogen-based drugs — are also more likely to experience morning sickness than less-nauseous individuals, Hoffman said. 

Thankfully, for most people afflicted with morning sickness, its symptoms tend to fade as the second trimester begins.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to [email protected] with the subject line "Health Desk Q," and you may see your question answered on the website!