Like many things during a pregnancy, the changes in a woman's senses are largely attributed to hormones. When women become pregnant, their levels of estrogen increase. Estrogen has been linked with an increased sense of smell in non-pregnant women as well. A 2002 study conducted by Philadelphia's Monell Chemical Senses Center showed that women of child-bearing age exhibited a greater sensitivity to odors than men. Groups with lower estrogen levels, like prepubescent girls and post-menopausal women, performed comparably with the men [source: Harvard Women's Watch].
So, as women's levels of estrogen rise and fall, their sense of smell may change. There's evidence that a woman's sniffer also changes during her menstrual cycle and at ovulation, as well as during pregnancy [source: Sample]. But scientists aren't exactly sure how (or if) estrogen creates the change in the nose or in the brain [sources: Harvard Women's Watch, Pletsch and Kratz].
Scientists also debate whether a pregnant woman's heightened senses serve any benefit to her or her unborn baby. Some researchers believe sensitivity to odors and taste causes morning sickness, benefiting the woman because she's rejecting foods containing chemicals and toxins harmful to the fetus [source: Nordin et al]. Scientists who subscribe to this theory say it explains why pregnant women are sensitive to the smell and taste of cigarettes, alcohol, bitter vegetables and caffeinated beverages such as coffee. Some data shows that women who experience nausea have a lower rate of miscarriage, suggesting that the nose is doing its job in keeping the baby safe [source: Stanford Report].
Other scientists would beg to differ. A 2004 study tested the hypothesis that because pregnant women have greater smell sensitivity, they would rate food items with toxins much more negatively. Did this turn out to be true? Not really. In the experiment, pregnant women didn't demonstrate any smell sensitivity at all. There was no evidence that they had higher smell sensitivity than non-pregnant women, and there were only small differences between women and men in general [source: Swallow et al]. Women also didn't rank the smells of the harmful products more negatively, and there was little correlation between smell rankings and nausea [source: Swallow et al].
The study's authors did mention that perhaps the nausea of morning sickness may come about when the product is actually tasted, not just smelled. A quarter of the women reported abnormal taste sensitivity in the early stages of pregnancy, including a heightened sensitivity to bitter items and a lowered sensitivity to salty items [source: Nordin et al]. Again, sensitivity to bitter items like coffee may be the body's way of protecting the unborn baby. Conversely, a decreased sensitivity to salt may help women to consume more salt, which in turn makes them thirstier and helps them consume the fluids and various nutrients they need to support the fetus [source: Nordin et al.].
Like changes in smell, changes in taste and why they happen have been hard for scientists to nail down. But when a cranky pregnant woman complains of these symptoms, it's probably best not to say there's no scientific evidence for what she's feeling. Instead, pregnant women should simply try to avoid the scents that aggravate them, which may involve allowing a spouse to do the cooking or politely asking a coworker to lay off the cologne. Pregnant women should try to leave the windows open when possible for ventilation, and they might see if the soothing scents of mint, lemon or ginger can offer them any relief [source: Murkoff].
Sniff around on the next page for other stories on pregnancy and the body.