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Stanford Report, September 20, 2000

New evidence connects nausea to sense of smell

Based on data from a pilot study, Stanford researchers hypothesize that the "morning sickness" many women experience during pregnancy may be connected to a heightened sense of smell.

Richard Blum, MD, and LeRoy Heinrichs, MD, both of gynecology and obstetrics at Stanford's School of Medicine, have co-authored a new book that surveys the literature on women's nausea and vomiting and presents new research to support this hypothesis. Blum, an emeritus consulting professor, and Heinrichs, active emeritus professor, have been invited to speak on the topic of nausea and vomiting in pregnancy at a Sept. 20-21 conference sponsored by the Pregnancy branch of the National Institute of Child Health and Human Development.

"Our research suggests that almost all of the stimuli for pregnancy nausea and vomiting are odors," Blum said, "probably as a result of the hyperacuity of the olfactory system induced by estrogen, which rises during pregnancy."

Data from earlier studies reveal that when women have nausea during pregnancy they are less likely to miscarry. The finding is thought to follow from the fact that a healthy placenta produces estrogen in the early part of a pregnancy; if the placenta is not producing estrogen -- and therefore not heightening the woman's olfactory sensitivity to the point of nausea -- then there is an associated problem with the pregnancy and thus a greater likelihood of miscarriage.

In their own research -- clinical studies at Stanford and formal surveys -- the authors found that women reported smell as a common trigger for nausea, and that a majority of pregnant women named smell as a precipitant to their nausea.

"Women have told us this for eons," said Heinrichs. "They say they can't stand the smell of cooking foods, particularly meats, bacon. They often state that the smell of coffee, perfumes, cigarette smoke, petroleum products -- anything volatile -- triggers their nausea."

Another study found that women born infertile and without a sense of smell -- anosmics -- suffered no nausea once they were helped to conceive and have normal pregnancies.

"Our hypothesis of olfaction as being important, if not the primary trigger, for nausea was supported," Heinrichs said. "But I emphasize that this is pilot data. We're developing hypotheses here that still need to be tested."

Heinrichs' and Blum's book, Nausea and Vomiting: Overview, Challenges, Practical Treatments and New Perspectives, was compiled by 20 students, 20 faculty members and one additional co-author. This first comprehensive book on nausea and vomiting emphasizes the experience of women. "Women suffer nausea and vomiting -- without regard to pregnancy -- at a rate three or four times greater than men," said Blum. "The majority of women rate it as one of the most painful experiences of their lives."

According to the researchers, few aspects of women's health have been as overlooked by researchers as the frequency and pain of women's nausea and vomiting, in pregnancy or otherwise.

"Women tend not to take their nausea and vomiting to a doctor," Blum said, "and probably for good reason, because there's not much a doctor can do." Physicians may prescribe an antiemetic to control vomiting, but these drugs are "notoriously poor at controlling nausea, which is the far more frequent and disabling experience," Blum added.

Research indicates that: 98 percent of women have experienced nausea, 97 percent vomiting; 43 percent have experienced nausea three times in the past six months, with half as many vomiting; 25 percent assign nausea to work stress, anxiety and nervousness; and half report nausea related to illness, surgery and dietary change.

Within their book the researchers suggest numerous potential paths for future research, including the study of such age-old remedies as ginger, which, anecdotally, has provided relief for patients suffering from extreme nausea. Heinrichs is hesitant to recommend ginger for nausea during pregnancy, though, because its active agent has been identified as a mitogen -- something that alters the genome and could affect the fetus.

In addition to his professional interest, Heinrichs had a personal interest that helped him realize the important role of smell in nausea. His son had for years suffered what appeared to be motion sickness on airplanes, with no relief provided by drugs. "One day we were taxiing," Heinrichs recalled, "and he started being nauseated before we took off. 'Don't you smell that awful smell?' he asked me. He meant the fuel. The next trip he took a piece of peppermint in his mouth before he boarded the plane, and now he flies all over with no trouble as long as he takes his peppermint." SR