Clinical Information: Progesterone’s primary function during the menstrual cycle is to induce a secretory endometrium ready for implantation of a fertilized egg. Levels therefore increase during the luteal phase of the cycle after ovulation. If no implantation occurs, progesterone returns to follicular phase levels. If a pregnancy results, progesterone continues to rise to very high levels and carries out a variety of functions necessary to sustain the pregnancy. In some patients with infertility, ovulation may occur but luteal phase levels of progesterone are inadequate. Luteal phase deficiency is a result of inadequate progesterone production by the corpus luteum. During menopause, ovarian progesterone production dwindles, resulting in postmenopausal levels similar to those seen in men. Progesterone has wide-ranging physiological effects, including neuroprotection, maintenance of skin elasticity, and development of bone tissue. Progesterone also counteracts the proliferative effects of estrogen on the endometrium. When samples are collected after transdermal application of progesterone, saliva progesterone levels are higher than serum, indicating distribution of progesterone to tissues. Reference range saliva progesterone levels in premenopausal women (luteal phase) are 75—250 pg/mL, and in postmenopausal women and men 12—100 pg/mL.