The several years, in the late forties or early fifties, when the great birth reservoir of potential ovarian follicles has been reduced to only a few, many with innately poor hormone-sensitivities (which is perhaps why they are still remaining...they never heard the clarion call of FSH). As fewer follicles are capable of fully- programmed function, corpus luteal fragilities start to show as diminished progesterone levels...later, even the pre-ovulatory estrogens start to diminish. The pituitary, sensing first the progesterone wobbles, then, maybe a year later, the erratic estrogens, tries to jump start the ovaries, sending increasing levels of Luteinizing Hormone (LH)...with diminishing results. Since the brain (hypothalamus) is actually controlling things, it is sending out higher levels of pituitary stimulating hormones, which the pituitary matches with its blood-carried trophic or gonadotropic hormones...in this case, LH.