Progesterone is a steroid hormone that plays a significant part in the reproductive system of many mammals. It is primarily synthesized in the ovaries by a temporary structure called the corpus luteum, which forms after ovulation. Smaller amounts of this hormone are also produced by the adrenal glands. The main function of progesterone is to prepare the uterine lining for the potential implantation of a fertilized egg and to regulate the timing of the menstrual cycle. This hormone is also important for maintaining a pregnancy once conception has occurred. Because its production is tied directly to the body’s reproductive state, a level considered “normal” changes dramatically depending on whether an individual is menstruating, pregnant, or post-menopausal.
Progesterone Levels Across the Menstrual Cycle
The concentration of progesterone fluctuates widely throughout the reproductive years, reflecting the different phases of the menstrual cycle. During the follicular phase, which starts on the first day of menstruation and lasts until ovulation, progesterone levels are naturally at their lowest baseline, usually less than 1 nanogram per milliliter (ng/mL) of blood.
The hormone’s concentration begins to rise slightly just before ovulation. Following the release of an egg, the remnants of the follicle transform into the corpus luteum, which becomes the primary source for progesterone production. This structure secretes large amounts of the hormone, causing a significant elevation.
This increase defines the luteal phase, during which progesterone works to thicken the endometrium, making the uterus receptive to an embryo. Progesterone levels in the mid-luteal phase generally fall between 5 to 20 ng/mL. If fertilization does not occur, the corpus luteum breaks down, causing a rapid drop in progesterone that triggers the start of menstruation.
Target Levels During Pregnancy
If conception takes place, the corpus luteum remains active under the influence of human chorionic gonadotropin (hCG), and progesterone levels continue to climb to support the developing pregnancy. In the first trimester, progesterone is essential for maintaining the uterine lining and preventing uterine contractions. Expected levels during the first 12 weeks of gestation typically range between 10 to 44 ng/mL.
As the pregnancy progresses into the second trimester, the placenta takes over the bulk of progesterone production from the corpus luteum. This shift is accompanied by a further increase in the hormone’s concentration in the bloodstream. Levels in the second trimester are generally found between 19.5 to 82.5 ng/mL, reflecting the placenta’s growing capacity.
By the third trimester, the placenta is producing substantial amounts of the hormone, which is necessary for uterine quiescence and preparing the mammary glands. Progesterone levels at this stage are at their highest, often falling within the range of 65 to 290 ng/mL. The maintenance of these high concentrations is fundamental to sustaining the pregnancy to term.
Interpreting High and Low Results
A progesterone result that falls outside the expected range for an individual’s physiological state can provide important diagnostic information. Low results in a non-pregnant individual during the luteal phase may suggest that ovulation did not occur (anovulation), or that the corpus luteum is not producing enough hormone (luteal phase defect). Persistently low levels are a common factor when investigating challenges with fertility.
In early pregnancy, a low progesterone level can be a significant indicator of a potential issue with the gestation. It may suggest a non-viable pregnancy, such as a threatened miscarriage, or it could be associated with an ectopic pregnancy where the embryo has implanted outside the uterus. A level under 10 ng/mL in the first trimester is often a strong concern that requires further clinical evaluation.
Conversely, higher-than-expected progesterone levels can also point to certain conditions. Naturally high levels outside of pregnancy are often attributed to the presence of functional ovarian cysts, which are temporary fluid-filled sacs that can produce hormones. Very elevated progesterone may be a sign of specific adrenal gland disorders, such as congenital adrenal hyperplasia, or certain types of ovarian tumors. During pregnancy, an extremely high result can sometimes be associated with a molar pregnancy or a multiple gestation, such as twins.
Baseline Levels and Testing Context
The lowest, most consistent progesterone levels are observed in individuals whose bodies are not actively undergoing the menstrual cycle or pregnancy. In post-menopausal women, the ovaries cease their cyclical function, and progesterone concentrations generally fall to less than 1 ng/mL. Similarly, men maintain a low, steady concentration of the hormone, typically measuring less than 1 ng/mL, as it serves as a precursor for other steroid hormones.
The timing of a progesterone test is a major factor in interpreting the result, especially when assessing ovulation. To accurately confirm that ovulation has occurred and to check the function of the corpus luteum, testing is ideally performed during the mid-luteal phase. For someone with a regular 28-day cycle, this typically falls around day 21.
However, the correct timing is actually seven days after ovulation, not a fixed calendar day, meaning the test date must be adjusted for shorter or longer cycles. Levels of progesterone can also experience minor fluctuations throughout the day, which means that a single measurement is considered a snapshot of the hormone’s concentration at that specific moment in time.