Progesterone is a steroid hormone primarily associated with the female reproductive system, playing a fundamental role in fertility and gestation. It is often referred to as the “hormone of pregnancy” due to its influence on the uterus and its sustained presence during gestation. Progesterone levels do not remain static but undergo constant, rhythmic changes over the course of a day, a month, and a pregnancy. Understanding these changes is complex because they are governed by multiple biological clocks operating simultaneously.
Progesterone’s Core Functions
Progesterone prepares the internal environment of the uterus for the potential arrival of a fertilized egg. It acts on the endometrium, causing the lining to thicken and become enriched with blood vessels and nutrients to support implantation. If conception occurs, the hormone maintains the integrity of the uterine lining, providing a healthy environment for the developing embryo.
Beyond reproduction, progesterone affects other body systems. It acts as a neurosteroid, influencing brain function, mood, and cognitive processes. It also helps prepare the mammary glands for lactation and prevents premature uterine contractions during pregnancy. Adequate progesterone levels are necessary for regulating the menstrual cycle and balancing the effects of estrogen.
The Answer: Daily Fluctuations and Circadian Rhythm
Yes, progesterone levels fluctuate throughout the day, a phenomenon controlled by the body’s internal 24-hour clock known as the circadian rhythm. The central regulator of this rhythm is the suprachiasmatic nucleus in the hypothalamus, which synchronizes the body’s processes with the external light-dark cycle.
The secretion of progesterone follows a diurnal pattern. Research suggests that the peak concentration, or acrophase, tends to occur in the morning, which is a common pattern for many hormones regulated by the circadian system. The lowest levels typically occur later in the day or during the night, though the precise timing can vary depending on the phase of the menstrual cycle.
This daily rhythm is part of a complex regulatory loop involving the hypothalamus, pituitary gland, and ovaries, known as the HPG axis. Circadian clock genes directly influence the production of steroid hormones within the ovaries. This regulation ensures the hormone is secreted in a pulsed, rhythmic fashion across the day. Disruptions to this daily rhythm, such as those experienced by shift workers, can potentially affect reproductive function.
Major Cyclical Changes in Progesterone Levels
While the daily rhythm exists, the most dramatic and clinically significant changes in progesterone occur over the monthly cycle in non-pregnant women. Levels are lowest during the follicular phase, which starts with menstruation and ends at ovulation, often remaining below 1 nanogram per milliliter (ng/mL).
A massive surge in progesterone begins immediately after ovulation, marking the start of the luteal phase. The temporary structure left behind on the ovary, called the corpus luteum, becomes the primary source. This exponentially increases progesterone levels to prepare the endometrium, reaching a peak of 5 to 25 ng/mL around seven days post-ovulation in a healthy cycle.
If the egg is not fertilized, the corpus luteum breaks down about 9 to 10 days after ovulation, causing a sharp decline in progesterone levels. This rapid drop triggers the shedding of the uterine lining, starting menstruation. If pregnancy occurs, the corpus luteum is rescued and continues production, with levels rising steadily throughout the first trimester. Production is then taken over by the placenta, which sustains concentrations, sometimes exceeding 300 ng/mL, to maintain the pregnancy.
Why Timing Matters: Implications for Testing
Understanding the daily and monthly fluctuations of progesterone is essential for accurate clinical testing and diagnosis. Because levels are constantly changing, a single blood test must be precisely timed to provide meaningful information about the patient’s hormonal status.
For monitoring ovulation and assessing the quality of the luteal phase, the standard practice is to test progesterone approximately seven days after ovulation. In a typical 28-day cycle, this corresponds to day 21, when the hormone is expected to be at its peak concentration. Testing at this peak time confirms that ovulation has occurred and that the corpus luteum is producing sufficient progesterone to support a potential pregnancy. For women with irregular cycles, the timing is adjusted to be seven days before the anticipated start of the next menstrual period.
The daily circadian rhythm means that blood should be drawn at the same time of day for comparative testing to minimize variability. Progesterone tests are used to investigate infertility, monitor early pregnancy health, and assess hormone therapies. A result above a threshold, often 10 ng/mL or 30 nmol/L, is considered evidence of healthy ovulation.