By Reagan Smith
We use our sense of smell every day, from enjoying food to detecting dangerous substances. Olfactory receptors enable us to perceive specific flavors in food, such as lemon, mint, coffee, and vanilla. Additionally, the receptors in our noses contribute to our perception of the environment around us.
One of the earliest signs of pregnancy is the development of a heightened sense of smell due to a sudden reduction of the individual’s olfactory threshold. Referred to as “hyperosmia,” this condition enables the subject to detect smells more easily, to display an increased reaction to perceived smells, and to identify specific odors with more ease.
It is determined by an “increased odor sensitivity during the past month compared to what is normal to that individual.” It can develop through a range of factors aside from pregnancy, including temporal lobe epilepsy, Addison’s disease, and migraines.
This reduced olfactory threshold provides an evolutionary advantage for the mother and the fetus in the womb. An increased sense of smell tends to act as a trigger for nausea and vomiting, otherwise known as morning sickness during pregnancy. As such, if the mother experiences a strong odor that indicates a possible ingested toxin, she will vomit, which, in turn, acts as a defence mechanism that protects the fetus from potentially harmful substances.
Interestingly, there are many self-reported accounts of a change in sense of smell in pregnancy. In fact, most of the evidence for hyperosmia in pregnant women comes from anecdotal evidence and surveys.
The earliest detection of this phenomenon was documented in 1895 by scientist H. Zwaardemaker, who conducted a study in Germany. In 1922, scientist G. Steiner observed in a study that nearly all pregnant women experience a stronger sense of smell, especially in the early months of pregnancy. A 1930 study concluded that due to pregnancy, the “sensitivity increased” and odors that were “normally imperceptible were now unbearable.”
These odors include cigarette smoke, cooked or spoiled food, coffee, gas, perfumes, and other strong smells, which contributes to the theory that hyperosmia develops as a defense mechanism against toxins. Those who were pregnant also displayed an ability to identify specific odors—rubber, rose oil, nitrobenzene (a bitter-tasting compound found in almonds)—which increased two to three days after pregnancy and stayed constant until two to three months after giving birth.
While concrete evidence of consistent, noticeable changes in olfactory perception of pregnant women is lacking (due to a number of factors), multiple minor studies and anecdotal accounts signify a correlation between pregnancy and hyperosmia. More research is needed to certify the hypothesis that it develops as protection for the fetus, but evidence tends to support this conclusion thus far.
Sources
Zwaardemaker H. (1895). Die physiologie des Geruchs. Leipzig: Engelmann
Steiner G. (1922). Psychische Untersuchungen an Schwangeren. Arch. Psychiatr. Nervenkr. 65 171–190 10.1007/BF02029573
Henssge E. (1930). Steigerung der Geruchsempfindlichkeit in der Schwangerschaft. Psychol. Med. 4 206–207
Cameron, E Leslie. “Pregnancy and Olfaction: a Review.” National Institute of Health, 6 Feb. 2014, www.ncbi.nlm.nih.gov/pmc/articles/PMC3915141/.
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