Why does pregnancy cause an increased sense of smell?

Many of those who are or have been pregnant claim that their sense of smell has increased in sensitivity.  Often, women claim they can smell foods and odors from a mile away.  In fact, a British retailer once employed pregnant women to smell wines and rate them due to their supposed heightened olfactory senses.  What is the truth behind this myth?  

Some researchers claim that the increased sensitivity is a defense mechanism for the fetus.  If a woman is turned off by an odor or smell, this serves as a warning for the mother to stay away from a particular food or substance.  In fact, this is one theory behind morning sickness.  If the mother’s body detects a substance that is perceived harmful to the fetus, then the mother is forced to rid the body of the substance.  Little data has validated this theory in empirical studies, although some researchers do agree there may be a correlation.  In a study investigating the correlation between olfactory senses and nausea, researchers found no difference in sensitivities to smell (Hummel, von Mering, & Koble, 2002).  

Another claim is that the heightened sense of smell is due to an increase in estrogen levels during pregnancy.  Studies have shown a positive correlation between estrogen levels and a heightened sensitivity to smell.  However, these findings are not limited to only pregnant women.  Women who are of post-pubescent age are generally able to detect smells better than men and post-menopausal women (Foley, 2010). 

Despite the claims of a heightened sensitivity to smell during pregnancy, researchers have found no evidence to support the claim that this is due to olfactory sensitivity.  In a study comparing olfactory performance between pregnant women and non-pregnant women, no difference was found between the two groups (Ochsenbein-Kolble, von Mering, Zimmermann, & Hummel, 2007).  In fact, the researchers found a slight decrease in sensitivity of pregnant women.  So why do two thirds of pregnant women claim they experience a higher sensitivity to smell during pregnancy?  The answer may lie in the cognitive processing of olfactory information that occur during pregnancy.  In the above studies, pregnant women seemed to differ in their “objective” sensitivity to smell rather than their “subjective” sensitivity, meaning pregnant women’s perception of increased sensitivity is due to changes in the brain (Kolble, et al, 2007).  More research is necessary to explain this phenomenon perhaps investigating the hormonal changes in the brain during pregnancy and how these changes perhaps “trick” the brain into believing pregnancy heightens the senses.

Elizabeth Brothers

Links:

http://tlc.howstuffworks.com/family/pregnancy-affect-senses-of-smell-and-taste.htm

References:

Edmonds, M. (2008). How does pregnancy affect the senses of smell and taste? How Stuff Works. Retrieved December 1, 2011 from http://tlc.howstuffworks.com/family/pregnancy-affect-senses-of-smell-and-taste.htm 

Foley, H. J., Matlin, M. W. (2010). Sensation and perception (5^th ed.). Boston: Pearson Education.

Hummel, T., von Mering, R., Kolble, N., 2002. Olfactory modulation of nausea during early pregnancy. International Journal of Obstetrics & Gynecology, 109, 1394-1397. doi: 10.1016/S1470-0328(02)02987-7

Kolble, N., von Mering, R., Zimmermann, R., Hummel, T., 2007. Changes in olfactory function in pregnancy and postpartum. International Journal of Gynecology & Obstetrics, 97, 10-14. doi: 10.1016/j.ijgo.2006.12.009