Progesterone is a steroid hormone essential to the female reproductive system. It is synthesized in the ovaries by the corpus luteum, which forms after ovulation. Smaller amounts are also produced by the adrenal glands, and during pregnancy, the placenta becomes a major source. Progesterone levels fluctuate widely depending on physiological state, providing insights into fertility, menstrual health, and pregnancy progression.
The Role of Progesterone in the Body
Progesterone prepares the body for pregnancy. Following ovulation, the hormone acts on the uterine lining, causing it to thicken and become enriched with blood vessels and nutrients.
If fertilization does not occur, the corpus luteum breaks down, leading to a sharp decline in progesterone levels. This drop signals the body to shed the prepared uterine lining, resulting in the menstrual period.
During early gestation, sustained progesterone is necessary to maintain the pregnancy until the placenta develops sufficiently to take over production. The hormone helps relax the uterine smooth muscle, preventing premature contractions. Progesterone also balances the effects of estrogen, preventing excessive build-up of the endometrium and regulating the overall reproductive cycle.
Reference Ranges Based on Physiological State
Progesterone concentration is typically measured in nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L), and the expected range varies significantly with the phase of the menstrual cycle or pregnancy.
During the follicular phase, which lasts until ovulation, progesterone levels are naturally at their lowest concentration, generally less than 1.5 ng/mL. This reflects that the progesterone-producing corpus luteum has not yet formed. Levels begin to rise only after the luteinizing hormone surge triggers ovulation.
The luteal phase, following ovulation, shows the highest non-pregnant progesterone levels. A mid-luteal phase measurement, taken about seven days after ovulation, typically ranges from 1.7 to 27.0 ng/mL. A level of 5 ng/mL or higher is commonly considered evidence of a functional corpus luteum, indicating successful ovulation.
In post-menopause, when ovarian function has ceased, progesterone production drops significantly to very low, steady levels, generally less than 0.8 ng/mL.
Progesterone levels in pregnant individuals increase progressively throughout the three trimesters. Concentrations in the first trimester range from approximately 8.8 to 48.6 ng/mL as the corpus luteum supports the early pregnancy. The second trimester sees a continued increase, typically between 12.4 and 75.8 ng/mL, as the placenta becomes the dominant hormone producer. By the third trimester, levels reach their peak, commonly rising to a range of 58.5 to 222.3 ng/mL, essential for uterine maintenance and fetal development.
Symptoms and Implications of Abnormal Levels
Low progesterone levels can affect menstrual cycle stability and fertility.
Low progesterone can manifest as irregular menstrual bleeding or spotting because the uterine lining may not be adequately maintained. It can also lead to a short luteal phase, where the time between ovulation and the next period is insufficient for a fertilized egg to properly implant. For those who become pregnant, low levels may signal an increased risk of miscarriage or ectopic pregnancy, as the hormone is needed to sustain the uterine lining.
A deficiency can also contribute to non-reproductive symptoms, including sleep disturbances, mood changes such as anxiety or depression, and hot flashes. Low progesterone can also lead to estrogen dominance, where the effects of estrogen are unbalanced, potentially causing heavy menstrual bleeding or weight gain.
Conversely, higher than normal progesterone levels in a non-pregnant person are less common but can produce noticeable symptoms. These elevated concentrations may cause side effects that mimic premenstrual syndrome. Common physical manifestations include:
- Excessive fatigue and drowsiness.
- Bloating.
- Fluid retention.
- Breast tenderness.
High progesterone levels, outside of pregnancy, may be associated with certain medical conditions. These include specific types of hormone-producing ovarian cysts. Congenital Adrenal Hyperplasia, a group of genetic disorders affecting the adrenal glands, can also result in elevated progesterone due to altered steroid hormone synthesis pathways.
Testing and Monitoring Progesterone Levels
Progesterone levels are most commonly measured through a blood serum test. This test is a valuable tool for confirming ovulation and evaluating the functional health of the corpus luteum.
The timing of the blood draw is a decisive factor, as the hormone concentration fluctuates throughout the cycle. For an accurate assessment of ovulation, the test is typically scheduled for the mid-luteal phase, generally seven days after ovulation or about seven days before the next expected period.
For individuals with a predictable 28-day cycle, this test is often referred to as a Day 21 progesterone test. For those with irregular cycles, the timing must be customized based on when ovulation is detected, often through tracking methods like basal body temperature or home ovulation predictor kits.
During fertility treatments or early pregnancy monitoring, testing may be repeated to ensure necessary hormone support is present. Results help guide decisions regarding progesterone supplementation to support conception or early pregnancy. While blood tests measure the circulating hormone (P4), some at-home methods use urine to measure the hormone’s metabolite, pregnanediol-3-glucuronide (PdG), as an indicator of progesterone activity.