The ovaries are the primary organ that produces progesterone in women. Specifically, a temporary structure inside the ovary called the corpus luteum is responsible for most progesterone production during the menstrual cycle. But the ovaries aren’t the only source. The placenta, the adrenal glands, and even the brain also produce progesterone under different circumstances.
The Corpus Luteum: Main Production Site
Each month after ovulation, the empty egg follicle in the ovary transforms into a small, hormone-producing structure called the corpus luteum. This structure is essentially a temporary gland, and its primary job is churning out progesterone during the second half of the menstrual cycle (the luteal phase). During this window, blood levels of progesterone rise from nearly undetectable (under 0.89 ng/mL in the first half of the cycle) to anywhere between 1.8 and 24 ng/mL.
The signal that triggers this production is luteinizing hormone, or LH, released by the pituitary gland in the brain. LH essentially flips the switch that tells the corpus luteum to start making progesterone. If no pregnancy occurs, the corpus luteum breaks down after about 10 to 14 days, progesterone drops sharply, and a period begins.
How the Placenta Takes Over During Pregnancy
If pregnancy does occur, the corpus luteum doesn’t shut down on schedule. A hormone produced by the early embryo, called hCG (the same hormone detected by pregnancy tests), keeps the corpus luteum alive and producing progesterone. This handoff is critical: the corpus luteum remains the near-exclusive source of progesterone through about the first six weeks of pregnancy.
Starting around week seven, the placenta begins producing its own progesterone, and a gradual transition takes place. By week ten, the placenta has fully taken over as the primary source. This timing matters because viable pregnancies are virtually never observed when progesterone levels fall below 5 ng/mL, and miscarriage occurs in roughly 80% of cases where levels drop below 10 ng/mL during early pregnancy.
Once the placenta is in charge, progesterone levels climb dramatically. First-trimester levels range from 11 to 44 ng/mL, second-trimester levels reach 25 to 83 ng/mL, and by the third trimester, levels can hit 58 to 214 ng/mL. These high levels keep the uterine lining stable and prevent the contractions that would otherwise end the pregnancy too early.
The Adrenal Glands
Both men and women produce small amounts of progesterone in the adrenal glands, the walnut-sized organs that sit on top of each kidney. Here, progesterone serves a different purpose. Rather than acting as a reproductive hormone, it functions mainly as a building block. The adrenal glands use progesterone as a chemical stepping stone to manufacture other hormones, including cortisol (the body’s main stress hormone) and aldosterone (which regulates blood pressure and fluid balance). The amount of progesterone that enters the bloodstream from the adrenals is relatively small compared to what the ovaries produce, but it’s the reason men and postmenopausal women still have detectable levels of the hormone.
Progesterone Made in the Brain
One of the more surprising production sites is the nervous system itself. Nerve cells and their supporting cells (called glia) can manufacture progesterone directly, without relying on supply from the ovaries or adrenals. Hormones made this way in the brain are called “neurosteroids” to distinguish them by their production site.
Brain-produced progesterone appears to play a protective role in nerve tissue. It supports the maintenance of the fatty insulation (myelin) that surrounds nerve fibers and influences how brain cells communicate with each other. The total pool of progesterone in the brain at any given time comes from three sources: what arrives through the bloodstream from the ovaries or adrenals, what the brain makes locally, and what gets converted into related active compounds within nerve tissue.
How All Progesterone Is Made
Regardless of where it’s produced, progesterone is built through the same two-step chemical process. The starting material is always cholesterol. An enzyme inside the cell’s energy-producing compartments (mitochondria) clips off a section of the cholesterol molecule to create an intermediate compound called pregnenolone. This is the rate-limiting step, meaning it controls how fast the whole process runs. A second enzyme then converts pregnenolone into progesterone. Every organ that makes progesterone, from the ovaries to the brain, uses this identical pathway.
Progesterone Levels Across Life Stages
Your progesterone levels vary enormously depending on your age, sex, and reproductive status. During the first half of the menstrual cycle, before ovulation, levels sit below 0.89 ng/mL. They spike during the luteal phase and peak again around ovulation (up to 12 ng/mL). After menopause, with the corpus luteum no longer forming each month, levels drop to 0.20 ng/mL or less, reflecting only the small contributions from the adrenal glands and other tissues.
Men maintain low but steady progesterone levels produced by the adrenal glands and, to a small extent, the testes. These levels are comparable to what women experience during the follicular phase or after menopause.