Women who do not use hormonal birth control pills experience a heightened sense of smell around the time of ovulation that women who are on the pill do not, according to a study published in Human Reproduction. As a result, Refinery 29 reports, pill-takers may be unwittingly mating with less-than-ideal specimen. According to another study, from the University of New Mexico, women who were not on the pill showed a preference for “the scent of men with symmetrical features” when they were most fertile. (Symmetry has a smell.) At least birth control users won’t be making any asymmetrical babies.