The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017

Pregnancy and Sense of Smell

Women have often reported that their sense of smell increases during pregnancy and that they are more sensitive to scent, particularly food and cooking odors. In fact, some women claim that this heightened sense of smell was their first indication that they were pregnant. Are there any scientific data to back up this anecdotal claim?

The Sense of Smell

The sense of smell, or olfaction, allows humans to differentiate between one trillion different odorants (Bushdid et al., 2014). Scent enters the nose and travels to the top of the nasal cavity where molecules enter the olfactory cleft. Receptors in the olfactory cleft send signals along nerve fibers to the olfactory bulb located in the brain. The olfactory bulb then relays signals to other parts of the brain, explaining why the sense of smell can be closely linked to emotion, memory, and learning.

Significance of Sense of Smell in Pregnancy

During pregnancy many women complain of increased sense of smell, sometimes called hyperosmia. This enhanced sense of smell could function to keep women away from “danger.” This danger could take many forms. Perhaps a woman’s heightened sense of smell will stop her from eating food that smells different due to bacterial or fungal contamination. Other sources of danger for the embryo could be in the form of chemicals or air pollution. Even body odor could alert a pregnant woman that a person is ill and therefore a potential danger to herself and her developing embryo.

Conflicting Evidence for Increased Sense of Smell During Pregnancy

Many studies report increases in women’s perceived sense of smell during pregnancy. A few studies, conversely, report that women rate their sense of smell as lower when pregnant. Self-reporting leads to inclusive and conflicting data because of the subjectivity of these reports and the variability of the questions asked and rating scales used. To address these inconsistencies, researchers have measured pregnant women’s ability to detect certain odors and compared these data to control groups consisting of women who were not pregnant and men. Again, the results vary depending on the odor tested.

Perhaps each woman becomes sensitive to different smells during pregnancy. Due to hormonal changes during pregnancy, a woman may become more easily conditioned to associate a smell with nausea and vomiting, making some odors seem stronger or more unpleasant. The close association between sense of smell and emotions and memory might protect humans from repeat experiences with foods that cause illness. Yet linking the memory of scents with nausea and vomiting could lead to increased nausea and vomiting when a woman is exposed to common scents. Linking the memory of a scent with the memory of nausea would certainly make a woman more aware of scents in her environment. Each woman is exposed to a different group of odors on a daily basis, which could explain the different sensitivities women demonstrate in different studies and the seemingly conflicting data revealed by many studies.

Estrogen, a hormone that fluctuates during a woman’s monthly cycle and then rises throughout pregnancy, has been hypothesized to cause increased sensitivity to smell. While estrogen does rise during pregnancy, sensitivity to smells seems to decrease after the first trimester of pregnancy. Estrogen may be working in combination with a variety of other hormones to alter a woman’s sense of smell during pregnancy, particularly during the first trimester when nausea and vomiting also seem to peak.

Future Research

Understanding the changes in the sense of smell during pregnancy will help us learn how the sense of smell works. If odor perception has measurable changes during pregnancy, further investigation could be undertaken to answer whether this change contributes to nausea and vomiting experienced during pregnancy. For some women, nausea and vomiting can become severe. Further research could determine whether aromatherapy or other alternative therapies could be effective in treating nausea and vomiting in pregnancy. Burning candles and sniffing lemons or herbs are common cures recommended to help with morning sickness; these cures may have a basis in fact due to the increased sense of smell experienced by some during pregnancy.

Lisa A. Rabe

See also: Anosmia; Dysosmia; Odor Intensity Scale; Olfactory Bulb; Olfactory Sensory Neurons; Perception

Further Reading

Bushdid, C., Marcelo O. Magnasco, Leslie B. Vosshall, & Andreas Keller. (2014). Humans can discriminate more than 1 trillion olfactory stimuli. Science, 343(6177), 1370—1372. http://dx.doi.org/10.1126/science.1249168

Cameron, E. Leslie. (2014). Pregnancy and olfaction: A review. Frontiers in Psychology, 5, 67. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3915141/

Gilbert, Avery N., & Charles J. Wysocki. (1991). Quantitative assessment of olfactory experience during pregnancy. Psychosomatic Medicine, 53, 693—700.