Progesterone Metabolite (PdG) is a measurable chemical marker found in urine, serving as an indicator of the body’s progesterone production. Progesterone is a steroid hormone that plays a major role in reproductive health by preparing and maintaining the inner lining of the uterus. Tracking the presence of PdG is a non-invasive way to confirm that the body has produced sufficient levels of progesterone after the monthly release of an egg. The eventual decline of this metabolite is a natural event that signals the conclusion of a reproductive cycle when pregnancy has not begun.
Understanding PdG and the Luteal Phase
PdG, or Pregnanediol Glucuronide, is the primary breakdown product of progesterone after the hormone is metabolized by the liver and then excreted. The source of this hormone is the Corpus Luteum (CL), a temporary structure that develops from the ovarian follicle immediately following the release of an egg. The presence of PdG in the urine confirms that ovulation has successfully occurred and that the CL is actively producing progesterone.
This period of progesterone production marks the start of the luteal phase. The function of this phase is to prepare the uterine lining, or endometrium, for the potential arrival and implantation of a fertilized egg. Progesterone facilitates this preparation by causing the endometrium to thicken and become enriched with blood vessels and nutrients. PdG levels subsequently rise in tandem with progesterone, often peaking around six to eight days post-ovulation (DPO) as the uterine environment is optimized for implantation.
The Physiological Timeline of PdG Decline
The PdG level begins its decline due to a programmed, self-destruct mechanism within the Corpus Luteum called luteolysis. This process is triggered by the absence of a specific hormonal signal that only a new pregnancy can provide. The CL has a relatively fixed lifespan, typically surviving for about 12 to 14 days after ovulation.
If a fertilized egg successfully implants into the uterine wall, it begins secreting Human Chorionic Gonadotropin (HCG), the hormone detected by pregnancy tests. HCG acts as a rescue signal, preventing the breakdown of the Corpus Luteum and prompting it to continue and even increase its progesterone production. In the absence of this HCG signal, the CL naturally starts to degrade, which causes its hormonal output to cease.
This degradation of the Corpus Luteum leads directly to a sharp reduction in progesterone, and consequently, a drop in its metabolite, PdG. For individuals who are not pregnant, the decline in PdG levels typically begins around 10 to 14 days post-ovulation. This timing is highly consistent across most cycles, as the luteal phase length is far less variable than the follicular phase.
The drop from peak PdG levels to baseline is often a rapid process, signifying the body’s shift out of the preparation stage. This decline occurs over a couple of days immediately preceding the onset of menstruation. Once the PdG level falls below the threshold required to sustain the uterine lining, the reproductive cycle concludes.
What the PdG Drop Signifies for the Body
The sudden withdrawal of progesterone, evidenced by the PdG drop, is the direct physiological trigger for menstruation. Without the hormone’s continuous support, the specialized lining of the uterus can no longer be maintained. The thick, blood-rich endometrial tissue is shed, initiating the menstrual flow and marking the first day of a new cycle.
This predictable hormonal event is useful for cycle tracking, as monitoring the PdG decline can accurately forecast the start of the next period. Individuals who track their PdG observe the point at which the hormone level returns to its pre-ovulation baseline, which correlates with the upcoming menstrual flow. The drop serves as a clear sign that the body is resetting its reproductive system to begin the preparation process anew.