Progesterone

Progesterone is a steroid hormone produced primarily by the corpus luteum in the ovaries after ovulation, and by the placenta during pregnancy. Along with estrogen, it is one of the two primary female sex hormones, though it is also produced in smaller amounts by the adrenal glands in both sexes. Progesterone's primary roles include preparing the uterine lining for potential pregnancy, maintaining early pregnancy, and regulating the menstrual cycle.

Beyond its reproductive functions, progesterone has calming effects on the brain (acting on GABA receptors to promote relaxation and sleep), supports bone density, influences thyroid function, and plays roles in cardiovascular health. It acts as a natural counterbalance to estrogen — while estrogen promotes cell growth and proliferation, progesterone modulates this growth and promotes cell differentiation. This balance is critical; unopposed estrogen (without adequate progesterone) increases the risk of endometrial hyperplasia and certain cancers.

Progesterone levels fluctuate significantly throughout the menstrual cycle — remaining low during the first half (follicular phase), surging after ovulation during the second half (luteal phase), and dropping sharply before menstruation if pregnancy does not occur. During perimenopause and menopause, progesterone production declines significantly, contributing to symptoms like irregular periods, sleep disturbances, anxiety, and mood changes.

Progesterone testing is commonly performed as part of fertility evaluations, menstrual irregularity workups, and pregnancy monitoring. A blood progesterone level drawn on day 21 of the menstrual cycle can confirm whether ovulation occurred. Low progesterone in early pregnancy may indicate increased risk of miscarriage.

In hormone replacement therapy (HRT) for menopausal symptoms, progesterone is essential for women who have a uterus — it counterbalances estrogen's stimulatory effects on the endometrium, reducing the risk of endometrial cancer. Micronized progesterone (bioidentical) is generally preferred over synthetic progestins due to a better cardiovascular and breast cancer risk profile.