Sensory Changes and Morning Sickness in Pregnancy

Pregnancy is a life changing event where one or possibly more offspring develop in the body of a woman. With great care and a certain amount of time, the offspring will grow and develop into a healthy infant child. During the 9-month period from fertilization to birth, many changes occur to the developing fetus. Week by week, expecting mothers check the progressive changes of their baby using different resources to see whether this week their baby is the size of an avocado or a mango. Not only are there changes to the baby’s body during this 9-month period, but also to the pregnant mother’s body. Examples of the most common changes seen during pregnancy are a change in hormone levels, enlarged breast tissue, constipation, and lower extremity edema just to name a few. Some changes are just as common but have limited scientific support. Some of those changes take place in the brain and the gastrointestinal tract.  The changes in the brain have to do with altered sensory perception, which include particularly olfaction and gustation, and also food cravings and aversion. These changes with the senses also are said to have an effect on the GI tract by causing nausea and vomiting, or Morning Sickness (Quinlan et al.  2003). This paper will explore the changes pregnancy has on the brain related to sensory perception, and how those changes affect the gastrointestinal tract.

A considerable amount of subjective data suggests that the sense of smell of an expecting mother during pregnancy is heightened. One study suggests that the reason for this increase in smell is due to an evolutionary advantage. The increased sensitivity allows women to sense any kind of harmful substance that may harm the mother or her fetus. (Simsek et al.  2015). Another study supports the suggestion that sensitivity is increased, but also suggests that odor identification is not, and may even be decreased during pregnancy (Cameron, 2007).Compared to non-pregnant women, expecting mother’s ability to identify different odors is not improved. Women in their first trimester were able to identify certain smells similar to that of non-pregnant women, but some smells were not able to identify while non-pregnant women were, suggesting that odor identification is not improved and may be decreased during pregnancy (Cameron, 2007). Although women cannot identify odors, women in their first trimester rated odors as more intense than non-pregnant women. A scratch-n-sniff test was done where both pregnant and non-pregnant women scratched a paper revealing an odor, where the more scratches released more odor. Pregnant women scratched significantly less times than a non-pregnant woman suggesting enhanced sensitivity to odors (Simsek et al.  2015). Also, pregnant women were asked if certain smells are more sensitive now than before they were pregnant, a vast majority of them reported smells that were strong and then, interestingly, commented on the pleasantness or unpleasantness of each smell. This study suggests that during pregnancy, a women is more able to distinguish a smell as more or less pleasant (Cameron, 2007). Although the science supporting olfactory changes during pregnancy are mostly subjective data, it still supports the theory that pregnancy causes a change in olfactory function.

The scent that goes hand in hand with olfaction and is also altered during pregnancy is the sense of taste, or gustation. A change in taste perception is experienced by a majority of pregnant women, 93% of pregnant women according to a recent questionnaire study (Ochsenbein-Kolble et al.  2005). A study reported by Nihon et al was performed to test gustatory function in the first, second, and third trimester. 32 pregnant women participated with 30 non-pregnant women as controls. Measurements of gustatory thresholds using a filter paper disk test showed that pregnant women had significantly higher gustatory thresholds than non-pregnant women, with a significant decrease in gustatory function in the first trimester (Ochsenbein-Kolble et al.  2005). Certain foods have been reported to taste differently, why these foods taste different during pregnancy is still unclear (Nihon et al.  1996). Intensity of certain foods has been reported to increase during pregnancy. They described an increase in bitter and salty intensity during the first trimester, and then a reduction in the second trimester. During the second and third trimesters, they reported that they preferred a higher concentration of salt in their food, possibly due to their low sensitivity to salt during that phase of pregnancy which would also confirm their finding that intensity decreases in the second and third trimesters. When asked which taste was preferred during pregnancy that was not a preferred test before pregnancy, majority of the pregnant reported sour to be a desired taste, followed by salty (Nihon et al.  1996). Interestingly, regarding the sweet taste, Similar to studies testing olfactory function testing odor identification, gustatory function in taste identification decreased significantly during pregnancy, especially in the first trimester. 

One major side effect of the changes olfaction and gustation is nausea and vomiting. Altered sense of smell and taste are not the primary cause of nausea and vomiting (Quinlan et al.  2003) but it may exacerbate symptoms a pregnant woman is already displaying. The etiology of nausea and vomiting during pregnancy is still unknown but many possibilities have been investigated. One suggestion is the altered level of the hormone Progesterone. One study prescribed non-pregnant women progesterone, which resulted in complaints of nausea, and altered gastric motility, suggesting the changes in progesterone levels to be a possible cause. The more common supported cause of nausea and vomiting by people is the presence of human chorionic gonadotropin, or hCG( Quinlan et al.  2003), but another study showed no change in nausea and vomiting with detected hCG, hence the roles of hCG remain controversial. As far as nausea exacerbation, studies show that the increased olfactory sensitivity may cause episodes of nausea and vomiting. A combination of increased olfactory sensitivity with the increased ability to identify smells as pleasant or unpleasant increases the chances to sense unpleasant smells at high intensity, leading to nausea and vomiting (Bayley et al.  2002). A positive outcome with decrease gustation function is it limits the ability of the taste of food to cause nausea and vomiting which preserves nutrient intake and contribute to safe blood volume. (Quinlan et al.  2003). It also allows women to eat more foods that otherwise would not taste as good but are beneficial to the developing fetus.

Changes in senses can lead to nausea due to the unpleasantness of the smell and sometimes taste. These changes also contribute to certain food cravings and aversions, and sometimes non-food cravings. Food cravings are said to be “all in your head”, and there is science that actually supports that, it is literally controlled by three parts of the brain, the hippocampus, insula, and caudate (Bayley et al. 2002). About 90% of pregnant women claim to have food cravings and aversions (Demissie et al. 1998) which occur primarily in the first and third trimesters. Monitored brain activity of pregnant women with food cravings and aversion showed activity in parts of the brain that are also related to pleasure and addiction, suggesting a largely related neurologic component to cravings and aversion. Pica, which is the desire to eat non-food substances, is actually very common (Bayley et al 2002). Examples include soil, laundry detergent, clay, and baking soda just to name a few. This affects gastric and intestinal motility which will affect nutrient absorption, in turn affecting nutrient intake of the developing fetus which may cause potentially serious harm. Further research concerning this issue is needed, especially to improve maternal and fetal health outcomes.

These are some of the many changes an expecting mother goes through during pregnancy and they have a huge impact on maternal and fetal health. Their ability to identify certain smells decreases but their sensitivity to the smell increases, as well as their ability to identify a smell as pleasant or unpleasant. Also, their sense of taste decreases in intensity, which may allow women to increase needed nutritious intake that they otherwise would not eat, thus contributing to the health of the developing fetus. Also it counteracts the increased olfactory function to control possible episodes of nausea and vomiting which can lead to loss of nutrients and blood volume in the mother. Understanding the changes in sensory perception during pregnancy and its effect on the body will allow us to know more how to control the negative effects of it and take advantage of the positive effects. This ultimately to improve maternal and fetal health, and make pregnancy a positive life changing event.

Works Cited

Bayley, T. et al (2002).  Food Cravings and Aversions During Pregnancy: Relationships with Nausea and Vomiting.  www.sciencedirect.com, p.  45-51.

Cameron, E.L., (2007).  Measures of Human Olfactory Perception During Pregnancy.  Oxford Journals, p.  775-782.

Demissie, T et al. (1998).  Food Aversions and Cravings During Pregnancy: Prevalence and Significance for Maternal Nutrition in Ethiopia.  Food and Nutrition Bulletin, : http://www.unu.edu/unupress/food/V191e/begin.htm#Contents.

Nihon, J., Kaiho, G., (1996).  A Study of Changes in Gustatory Sense During Pregnancy.  PubMed, 1208-17.

Ochsenbein-Kolble, N., Hummel, T., Von Mering, R., Zimmerman, R., (2005).  Changes in Gustatory Function During the Course of Pregnancy and Postpartum. US National Library of Medicine, PubMed.

Quinlan, J.D., Hill, D.A., (2003).  Nausea and Vomiting of Pregnancy.  American Family Physician, Florida Hospital, 121-128.

Simsek, G.,  Bayar Muluk, N.,  Arikan, O.K.,  (2015).  Marked Changes in Olfactory Perception During Early Pregnancy: A Prospective Case-Control Study.  European Archives of Oto-Rhino-Laryngology, p.  627-630