Following birth, estrogen and progesterone levels drop because of the loss of the placenta, and therefore negative feedback on the hypothalamus and pituitary imposed by these hormones during pregnancy is gone. Nevertheless, women who breast feed their infants may not have their ovarian and menstrual cycles return for some time—a phenomenon called lactational delay. In Figure 40.4 you learned that the pituitary hormone prolactin stimulates the growth of the mammary glands and the production of milk. Prolactin release is stimulated by the suckling of the infant. Even the sight or sound of the infant can stimulate prolactin release in the mother. Investigations have now shown that in addition to stimulating the mammary glands, prolactin also inhibits GnRH release and LH release, possibly explaining lactational delay. Going back to the case of the tammar wallaby in the opening to this chapter, could prolactin release stimulated by the suckling of the joey in the pouch be responsible for the delayed implantation seen in this species? Investigating Life: The Control of Diapause in the Tammar Wallaby examines this question.