Effects of breastfeeding chemosignals on the human menstrual cycle

Background: To date, there has not been an investigation to determine whether lactating women and their infants influence the ovarian function of other women with whom they interact. Methods: In a randomized, double-blind, placebo-controlled study of 47 nulliparous women, we utilized both within- and between-subjects controls to assess the effects of sustained exposure to breastfeeding compounds on menstrual cycle length, as well as characteristics of each phase of the ovarian cycle. Results: Breastfeeding compounds modulated ovarian cycle length in comparison with the carrier control (0.01 ≤ all P values ≤ 0.05), disrupting the normal homeostatic regulation of cycle length and tripling its variance. Hence, women with long cycles stayed long and did not regress to the mode of 29 days and women with short cycles maintained short cycles. This effect was driven by changes in both the follicular and luteal phases of the cycle (0.01 ≤ all P values ≤ 0.04) and changed the timing of the pre-ovulatory surge of LH. Conclusions: Because compounds from lactating women and their infants modulated the ovarian cycles of women, as is seen in other mammals, they have the potential to function as pheromones, regulating fertility within groups of women.