| When our ovaries begin gently signaling that they do not want to continue the task of producing an egg every cycle, they don't do it by dropping estrogen levels. Instead, we start to experience the odd cycle when no egg is produced. In those months, the ovaries do not produce progesterone. And the relation between estrogen and progesterone levels in our bodies begins to change. We know now that it's progesterone that affects our hearts, our blood vessels, our nerves, our brains and even our estrogen activity. Because progesterone does so much more than manage fertility, its decline can produce a lot of dramatic effects. We experience these effects as hot flashes, anxiety, sleeplessness, changes in sexual energy and vaginal changes. Today we understand that it's progesterone—that other female hormone—that is missing, causing all kinds of subtle imbalances by its absence. And that led us to believe that supplementing the circulating levels of progesterone would curb a whole range of distressing experiences women had simply come to accept as part of being a woman.
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