Women prefer slightly feminized male facial shapes. Such faces (Fig. 1a) are given positive personality attributions that might correlate with actual behaviour. In contrast, masculine features seem to signal immunological competence. Heritable benefits can be realized only if conception follows copulation, so women might be more attentive to phenotypic markers indicating immunological competence during the follicular phase of the menstrual cycle when conception is most likely,. Consistent with this hypothesis is the observation that women's preference for the odour of men with low fluctuating asymmetry (a correlate of testosterone-facilitated trait size and developmental stability) increases with the probability of conception across the menstrual cycle.Symmetrical men report more extra-pair copulation partners, and extra-pair copulation rates peak in midcycle. Here we show that female preference for secondary sexual traits in male face shapes varies with the probability of conception across the menstrual cycle.