Commitment to relationships and preferences for femininity and apparent health in faces are strongest on days of the menstrual cycle when progesterone level is high.
Jones BC, Little AC, Boothroyd L, Debruine LM, Feinberg DR, Smith MJ, Cornwell RE, Moore FR, Perrett DI. Horm Behav. 2005 Sep; 48(3):283-90.