Progesterone is a crucial steroid hormone that plays a significant role in the female reproductive system, acting as a chemical messenger to regulate various bodily functions. It is primarily produced in the ovaries by the corpus luteum, a temporary structure that forms after ovulation, with smaller amounts synthesized by the adrenal glands. The main biological function of progesterone is to prepare the endometrium, which is the lining of the uterus, for the potential implantation of a fertilized egg. This hormone is also responsible for maintaining the early stages of a pregnancy and is sometimes referred to as the “hormone of pregnancy.”
Measuring Progesterone
Progesterone levels are typically measured using a serum test, with results reported in nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L). Because progesterone levels fluctuate dramatically throughout the menstrual cycle, the timing of the blood draw is fundamental for accurate interpretation. Testing is often scheduled approximately seven days after ovulation (day 21 of a 28-day cycle) to assess whether ovulation has occurred. This timing allows clinicians to measure the peak concentration produced by the corpus luteum, providing a clear indication of its function.
Standard Levels in Non-Pregnant Individuals
The concentration of progesterone varies widely in non-pregnant individuals depending on the specific phase of the menstrual cycle. During the follicular phase (the time before ovulation), levels are at their lowest baseline, typically measuring less than 1 ng/mL (under 3.18 nmol/L). This low level reflects minimal hormone production before the release of an egg.
Following ovulation, the corpus luteum forms and begins to secrete substantial amounts of progesterone, causing a sharp rise in concentration for the luteal phase. A mid-luteal phase concentration confirming successful ovulation generally falls within the range of 5 to 20 ng/mL (15.90 to 63.60 nmol/L). A level above 10 ng/mL is considered a strong indicator of adequate corpus luteum function, which is necessary to support a potential pregnancy.
In post-menopausal individuals, the ovaries cease regular function, leading to a significant drop in hormone production. Levels return to a low baseline, remaining below 1 ng/mL. Males also maintain very low concentrations, typically less than 1 ng/mL, as the hormone is produced only in small amounts by the adrenal glands.
Progesterone Levels During Pregnancy
Once conception occurs, progesterone levels continue to rise sharply and reach concentrations far exceeding those seen during the luteal phase of a non-pregnant cycle. During the first trimester, concentrations are broad, generally falling between 11.2 and 90.0 ng/mL. This high concentration is initially maintained by the corpus luteum, which remains active to sustain the uterine lining.
As the pregnancy progresses, the placenta begins to develop and eventually takes over the primary role of progesterone production. This shift is accompanied by a further increase in circulating hormone levels. The second trimester sees levels climb to a range of about 25.6 to 89.4 ng/mL.
The third trimester marks the peak of progesterone production, with concentrations often ranging from 48 to 300 or more ng/mL. These extremely high quantities maintain uterine quiescence by suppressing muscular contractions, thereby preventing preterm labor. Additionally, this sustained elevation prepares the mammary glands for milk production.
Therapeutic Use and Dosing
Progesterone is widely administered therapeutically to supplement naturally occurring levels or to counteract the effects of other hormones. For women undergoing Hormone Replacement Therapy (HRT) who still have a uterus, progesterone is prescribed to protect the endometrial lining from the stimulating effects of estrogen. A common approach involves oral micronized progesterone at a dose of 200 milligrams (mg) taken daily for 12 to 14 days of a 28-day cycle, or a continuous daily dose of 100 mg.
In fertility treatments, such as Assisted Reproductive Technology (ART), progesterone supplementation supports the luteal phase and ensures the uterine lining is receptive for embryo implantation. The dosage for this application is frequently delivered directly to the target tissue using vaginal suppositories or gels. A typical regimen involves a vaginal insert dose of 100 mg administered two to three times a day, starting the day after egg retrieval.
Other therapeutic applications include addressing amenorrhea, or the absence of a menstrual period. For this, a higher oral dose of 400 mg daily for 10 days may be used to induce withdrawal bleeding. The specific quantity of administered progesterone is carefully calibrated to achieve a local or systemic physiological effect, such as preparing the uterus or protecting the endometrium. Treatment focuses on the administered dose rather than the resulting blood concentration.