Pregnanediol Glucuronide (PDG) is the primary urinary marker used to track the body’s production of the hormone progesterone. Progesterone is metabolized by the liver into PDG, a water-soluble metabolite that is then excreted in the urine. Measuring this metabolite provides a non-invasive way to monitor progesterone status, especially for those trying to conceive or in early pregnancy. This article explains the function of progesterone during gestation, how PDG testing works, and the expected PDG levels during a healthy pregnancy.
Progesterone: The Hormone of Pregnancy
Progesterone is often referred to as the “pregnancy hormone” due to its role in establishing and maintaining gestation. Its initial function is to prepare the endometrium, the inner lining of the uterus, for the attachment of a fertilized egg. The hormone causes the uterine lining to thicken and enrich it with blood vessels and nutrients to support the developing embryo.
Once implantation occurs, progesterone maintains the uterine environment by preventing the shedding of the endometrial lining, which would otherwise result in menstruation. It also decreases the contractility of the uterine smooth muscle. This relaxing effect helps prevent premature contractions.
Initially, the corpus luteum produces progesterone. Around the 8th to 10th week of gestation, the placenta takes over this function, dramatically increasing production. This hormone also helps regulate the maternal immune response, preventing the mother’s body from rejecting the developing fetus.
PDG: The Metric for Monitoring Progesterone
Progesterone is a steroid hormone that circulates in the bloodstream. Before elimination, it is metabolized, primarily by the liver, into pregnanediol. This is then conjugated with a glucuronide molecule to become PDG, which is excreted in the urine.
Testing for PDG in urine offers a significant benefit over traditional serum progesterone blood tests. A serum test captures only a single snapshot of the hormone level at the moment the blood is drawn. Progesterone is released in pulsatile bursts, meaning its levels can fluctuate dramatically, sometimes up to eightfold, over a single day.
Urinary PDG testing, especially using a first-morning urine sample, offers a more complete picture of hormone activity. The PDG level in that sample reflects the average systemic progesterone level from the previous 24 hours. This smoothing of the daily fluctuation makes PDG a reliable marker for monitoring overall progesterone status over time.
PDG and serum progesterone levels are directly correlated, but their measurements use different units and numerical values. For example, a serum progesterone level of approximately 10 nanograms per milliliter (ng/mL) is roughly equivalent to a PDG concentration of 5 micrograms per milliliter (µg/mL) in urine. This difference must be considered when interpreting results from at-home urinary tests versus lab-drawn blood work.
Interpreting PDG Levels Throughout Gestation
Progesterone and PDG levels increase significantly once pregnancy is established. In early pregnancy, sustained PDG elevation above the pre-ovulation baseline is an early sign of a viable pregnancy. Many at-home tests use a threshold of 5 µg/mL to confirm successful ovulation, but levels often rise above 10 µg/mL in the first few weeks of a successful pregnancy.
Since PDG is a metabolite, universal reference ranges are not as standardized as those for serum progesterone and can vary between testing brands and laboratories. Monitoring the general trend of progesterone production provides the best framework for understanding normal gestation. Levels are highest during the first trimester when the corpus luteum is responsible for production, and they continue to climb as the placenta matures.
For serum progesterone, which directly informs PDG levels, the expected ranges rise over the course of the three trimesters. In the first trimester, serum levels generally range from approximately 9 to 47 ng/mL. This wide range reflects individual variability and the stage of placental development.
The second trimester sees a rise in serum levels, typically ranging from 17 to 146 ng/mL. The third trimester is marked by the highest concentrations, ranging from 49 to over 300 ng/mL. A single reading is less informative than a trend; a healthy pregnancy shows a steady, continuous increase in levels over weeks.
The context of the measurement is important, as a level considered high in the first trimester may be suboptimal in the third. Fluctuations are expected, but a consistent upward trend indicates robust placental hormone support. Always compare results to the specific reference range provided by the laboratory or test manufacturer.
Addressing Low PDG Levels
Consistently low PDG levels during early pregnancy may indicate inadequate progesterone support for the uterine lining. This situation is associated with an increased risk of complications, such as threatened miscarriage or ectopic pregnancy. However, low progesterone is often a consequence of a non-viable pregnancy rather than the cause of the loss.
If low progesterone is suspected to be a contributing factor, a healthcare provider may suggest supplementation. Progesterone can be administered via oral capsules, vaginal suppositories, or injections. The vaginal route is often preferred because it delivers the hormone directly to the uterus, minimizing systemic side effects.
Supplementation is commonly used for individuals undergoing in vitro fertilization (IVF) because the procedure bypasses the corpus luteum’s natural function. For spontaneous conceptions, supplementation is reserved for those with a history of recurrent pregnancy loss or specific medical indications. The goal is to ensure the uterine environment remains stable until the placenta produces the necessary levels independently.
Self-treating based on at-home PDG results is not advisable, as interpretation requires professional medical context. Concerns about low PDG should be confirmed with a healthcare provider. They can order a serum progesterone test and other diagnostic tools, such as an ultrasound, to determine the appropriate course of action.