The sense of smell in women who are on the pill is tuned in to food, while women who are not on the pill are better at picking up the odor of male sweat, according to researchers at Sweden's Uppsala University.

And that's not all. Birth control pills change the type of male face that women find attractive say British researchers who have discovered that women on the pill have different ideals of male sexual attractiveness, compared with those who are not taking oral contraceptives.

It has long been known that the use of hormones affects the sense of smell in humans, but this influence has been seen as general, not specific. That is, the sensitivity to all smells is impacted to the same extent. In this study, the researchers measured the sensitivity of women to two odors, one that signals food (nourishment) and one considered to be a social smell, a pheromone that occurs naturally in male sweat.

Women who were taking birth control pills were more sensitive to the smell signaling nourishment, but less sensitive to the social smells. Moreover, this difference in sensitivity to smells seems to be linked to the degree of fertility. Women in the most fertile phase of their menstrual cycle were the most sensitive to the male sweat substance, notes study leader Johan Lundstrom. The study findings were published in the journal of Biological Psychology.

Birth control pills have another unusual side effect: They change the type of male face that women find attractive, reports London's Times Online of research conducted by psychologists at the Perception Lab located at the University of St. Andrews in Scotland. Why does this happen? The pill mimics the hormonal effects of pregnancy. That means women who are pregnant or on the pill show particularly strong preferences for male faces with a healthy glow and shun men with pallid, pasty features. The researchers think it's due to the female hormone progesterone.

When a woman becomes pregnant, her levels of progesterone rise, and this may trigger an internal mechanism that has evolved to protect expectant mothers against contact with disease, the researchers said. In other words, men who look healthy and strong are appealing; men who look ill are not. This has nothing to do with romantic attraction; rather, it's an innate desire for the pregnant woman to stay healthy herself--and keep her unborn baby healthy, too. The study findings were published in the Proceedings of the Royal Society.