Progesterone is a naturally occurring steroid hormone that plays a fundamental role in the reproductive system, primarily preparing the body for potential pregnancy. It is produced mainly by the corpus luteum, a temporary structure in the ovary that forms after ovulation. Progesterone’s main function is to transform the inner lining of the uterus (the endometrium), making it receptive to a fertilized egg and supporting implantation. If conception occurs, the placenta takes over progesterone production, maintaining the uterine environment and preventing contractions. Smaller amounts are also produced by the adrenal glands in both females and males.
How Progesterone Levels Are Measured
The most common method for quantifying progesterone is a serum blood test, which measures the concentration circulating in the bloodstream. Because levels fluctuate dramatically over the menstrual cycle, timing is a careful consideration. The test is typically performed during the mid-luteal phase, approximately seven days after ovulation, often called a “Day 21” test on a standard 28-day cycle.
This timing is important because it is when progesterone is expected to be at its peak, indicating whether ovulation successfully occurred. Levels are commonly reported in nanograms per milliliter (ng/mL), though some laboratories may use nanomoles per liter (nmol/L). While serum tests are the clinical standard, other methods exist, such as testing progesterone metabolites in urine or measuring the hormone in saliva for at-home tracking.
Typical Progesterone Ranges by Life Stage
Progesterone production varies significantly across a person’s reproductive lifespan, reflecting the body’s current hormonal demands. During the follicular phase of the menstrual cycle, which runs from the first day of menstruation until ovulation, levels remain low, typically measuring less than 1.5 ng/mL. This low concentration allows for the maturation of an egg follicle, a process driven primarily by other hormones.
Following ovulation, the corpus luteum begins to secrete progesterone, leading to a sharp increase during the luteal phase. In a non-pregnant cycle, levels generally reach a peak range of 2 ng/mL to 25 ng/mL, with values greater than 5 ng/mL often confirming ovulation. If pregnancy does not occur, the corpus luteum degenerates, the level drops, and menstruation is triggered.
If conception is successful, the corpus luteum is sustained, and progesterone levels continue to climb throughout gestation. During the first trimester, levels typically fall between 10 ng/mL and 44 ng/mL. The concentration continues its upward trend in the second trimester, with expected ranges spanning from approximately 19.5 ng/mL to 82.5 ng/mL.
The highest levels are reached in the third trimester, as the placenta takes over production, with concentrations commonly reported between 65 ng/mL and 290 ng/mL or higher. This sustained high level is necessary to maintain the uterine lining, prevent premature contractions, and prepare the body for milk production. Post-menopause, the ovaries cease cyclical production, resulting in perpetually low levels, usually less than 1 ng/mL.
Health Implications of Deviant Levels
When progesterone levels fall outside the expected ranges, it can indicate underlying reproductive or endocrine issues. Low progesterone outside of pregnancy is often associated with a short or dysfunctional luteal phase, leading to irregular menstrual periods or difficulty conceiving. This deficiency may prevent the uterine lining from becoming adequately prepared for implantation or cause it to shed prematurely.
If low levels occur during early pregnancy, it raises concerns about the health of the gestation, often correlating with an increased risk of miscarriage or ectopic pregnancy. Chronically low levels can also manifest as premenstrual symptoms like anxiety, mood changes, and sleep disturbances, reflecting progesterone’s role in the nervous system.
High progesterone levels are most often a natural and expected occurrence, such as during the peak of the luteal phase or throughout pregnancy. However, when not pregnant, unusually high readings can occasionally signal a less common condition, such as certain types of ovarian cysts or adrenal gland disorders. Symptoms that may accompany a high, non-pregnancy-related level include excessive fatigue, bloating, and breast tenderness, which can mimic intense premenstrual syndrome.
Therapeutic Uses of Progesterone
Progesterone, administered either in its bio-identical form or as a synthetic version (progestin), is a common medical intervention. One major application is in Hormone Replacement Therapy (HRT) for women with an intact uterus receiving estrogen therapy for menopausal symptoms. Progesterone is added to counteract the proliferative effect of estrogen on the uterine lining, reducing the risk of developing endometrial hyperplasia or cancer.
In fertility medicine, exogenous progesterone is frequently used to support the luteal phase, particularly following in-vitro fertilization (IVF), aiding in embryo implantation and maintenance of the uterine lining. It is also prescribed to women with a history of recurrent miscarriage to strengthen the pregnancy environment. Beyond reproductive health, progestins are incorporated into contraceptive methods, such as birth control pills or injections, where they primarily suppress ovulation and thicken cervical mucus.