Savic 2001b found that men are most attracted to the scent of women who are genetically different from them. And according to Lundstrom 2006, women who are on the "pill" or  hormonal birth control do not make the same pheromones or have the ovulatory-phase rise in testosterone derivatives like androstenedione that stimulate the sweat glands to produce those feminine "come-hither" pheromones.