Progesterone

Progesterone is one of the two primary female sex hormones — the other being estrogen — and it plays a central role in the second half of the menstrual cycle. After ovulation, the empty follicle (the structure that held the egg) transforms into a temporary gland called the corpus luteum, which produces a surge of progesterone. This hormone thickens and prepares the uterine lining to receive a fertilized egg.

If fertilization doesn't occur, progesterone levels drop, the uterine lining sheds, and menstruation begins. If pregnancy does occur, the corpus luteum continues producing progesterone — and later, the placenta takes over — to maintain the uterine environment and support the developing pregnancy. Progesterone also plays a role in breast development, sleep regulation, and mood.

Outside of reproductive health, progesterone is an important component of hormone replacement therapy (HRT) for menopausal women who still have a uterus. Because estrogen alone can cause overgrowth of the uterine lining, progesterone (or a synthetic version called progestin) is added to protect against that risk. Understanding the balance between estrogen and progesterone is central to many aspects of women's hormonal health.

Progesterone affects far more than fertility. Its presence and balance relative to estrogen influences mood, sleep quality, anxiety levels, and the regularity of the menstrual cycle. Low progesterone can contribute to irregular periods, PMS, difficulty conceiving, and early pregnancy loss — making it a hormone worth paying attention to if any of those issues are present.

In breast cancer, tumor cells are tested for progesterone receptors. Cancers that are progesterone receptor-positive (PR+) tend to grow in response to the hormone and are generally more responsive to hormone-blocking therapies. This single piece of pathology information helps guide a meaningful treatment decision, illustrating how central progesterone is not just to reproductive health but to cancer care as well.