Progesterone is a steroid hormone that regulates the menstrual cycle and prepares the female body for potential pregnancy. It is often referred to as the “pregnancy hormone” because of its role in sustaining a viable gestation. A woman’s progesterone level is not a single fixed number but fluctuates dramatically based on her current physiological state. Understanding the expected range requires knowing the woman’s current physiological state, as what is normal in one phase can be considered highly abnormal in another.
Measuring Progesterone Levels
Progesterone levels are most commonly measured through a blood test, which reports the concentration of the hormone in the serum. The standard unit of measurement is nanograms per milliliter (ng/mL) of blood. This test is frequently ordered to determine if ovulation has occurred or to assess the health of an early pregnancy. The timing of the test is crucial for accurate interpretation, especially when assessing ovulation in a non-pregnant woman. To confirm a healthy ovulatory cycle, the test is typically performed during the mid-luteal phase, approximately seven days after ovulation. In a standard 28-day cycle, this corresponds to around day 21, which is why it is often called a “day 21 progesterone test.” If a woman has an irregular cycle, she may be instructed to have the blood draw exactly seven days before her next expected period.
Progesterone Levels Across the Menstrual Cycle
Progesterone levels are at their lowest during the follicular phase, which starts with menstruation and ends at ovulation. During this time, the reference range is generally less than 1 ng/mL, as the ovary is focusing on developing a dominant follicle.
The level remains low until after ovulation, when the ruptured follicle transforms into the corpus luteum. This structure immediately begins to secrete large amounts of progesterone, causing a sharp rise in concentration. The luteal phase is characterized by these elevated levels, which typically range from 2 to 25 ng/mL. A measurement over 5 ng/mL often indicates successful ovulation, as this high level is necessary to maintain the uterine lining for potential implantation. If conception does not occur, the corpus luteum breaks down, triggering menstruation.
Progesterone levels in women who have gone through menopause are also consistently low, typically less than 1 ng/mL. This is because the ovaries have ceased their cyclical function.
Progesterone Levels During Pregnancy
When a fertilized egg implants in the uterine wall, the corpus luteum continues to produce progesterone, and the levels remain high to support the pregnancy. Progesterone is fundamental in early gestation, as it maintains the uterine lining and prevents the uterus from contracting prematurely. The expected ranges increase significantly during pregnancy, and there is a wide range of what is considered normal.
During the first trimester, progesterone levels can range from approximately 9 to 47 ng/mL. The hormone is initially produced by the corpus luteum, which is sustained by the pregnancy hormone human chorionic gonadotropin (hCG). As the pregnancy progresses, the placenta gradually takes over progesterone production.
By the second trimester, the ranges typically expand to about 17 to 147 ng/mL. In the third trimester, the levels continue to rise, often reaching a range of 55 to 300 ng/mL or more. This continuous increase ensures the pregnancy is maintained until full term, with the high levels also preparing the mammary glands for lactation.
Interpreting Abnormal Results
A progesterone result falling outside the expected range for the woman’s cycle phase or pregnancy status provides important diagnostic information. Abnormally low levels in a non-pregnant woman during the luteal phase may indicate anovulation, meaning the ovary failed to release an egg, or a luteal phase defect. Low levels are also associated with irregular menstrual periods and can signal conditions like Polycystic Ovary Syndrome (PCOS).
In early pregnancy, a low progesterone result can raise concern for an increased risk of miscarriage or an ectopic pregnancy, though a single low value is not definitive. Since progesterone is essential for maintaining the uterine lining, insufficient levels can compromise the embryo’s ability to thrive. Low levels may prompt a doctor to check for other issues, such as hypothyroidism or high levels of the hormone prolactin.
Conversely, a progesterone level that is higher than expected when a woman is not pregnant is a less common occurrence. Elevated levels may be a sign of certain types of ovarian cysts or an inherited condition called congenital adrenal hyperplasia. Very high levels during pregnancy, though often normal, can sometimes be associated with a multiple gestation pregnancy or a molar pregnancy.