Estrone-3-glucuronide, or E3G, is a substance derived from estrogen and found in urine. As the body breaks down hormones for removal, it creates E3G. While estrogen is known for its role in the reproductive system, E3G serves as a urinary marker that reflects the amount of estrogen in the bloodstream. This allows individuals to monitor aspects of their reproductive health without invasive blood tests, as its concentration provides information at different stages of a person’s reproductive life.
The Origin and Function of E3G
E3G originates from estradiol (E2), a primary form of estrogen found in the blood. For the body to eliminate estradiol, it must be metabolized by the liver. During this process, the body converts estradiol into estrone. A molecule called glucuronide is then attached to estrone, creating estrone-3-glucuronide (E3G), which makes the hormone water-soluble. This allows it to be filtered by the kidneys and excreted in urine, enabling non-invasive testing since urinary E3G levels correlate with blood estradiol levels.
E3G in the Menstrual Cycle
The concentration of E3G in urine changes predictably throughout the menstrual cycle, following the rise and fall of estrogen. During the follicular phase, a developing follicle in the ovary produces increasing amounts of estradiol. This increase in estradiol leads to a corresponding rise in urinary E3G, which signals that the body is entering its fertile period. The rising estrogen prepares the uterus for a potential pregnancy and alters cervical mucus to become more hospitable to sperm. The E3G peak occurs just before the hormone surge that triggers ovulation, after which E3G levels will drop.
Measuring E3G for Fertility Tracking
The predictable pattern of E3G makes it a useful marker for fertility tracking with at-home test kits. These devices detect the concentration of E3G in a urine sample, establishing a baseline early in the cycle and then identifying a significant increase. This rise in E3G indicates the fertile window has opened, which happens one to three days before ovulation.
These tracking systems often measure a second hormone, Luteinizing Hormone (LH). While the E3G surge identifies the start of the fertile window, the LH surge pinpoints the peak fertile days. The LH surge is the direct trigger for the ovary to release an egg, an event which occurs 24 to 36 hours after the surge is detected. Monitoring both E3G and LH helps identify a wider fertile window of up to six days, enhancing the chances of conception.
Using both hormones provides a more complete picture than measuring LH alone. E3G offers an advanced warning that ovulation is approaching, while LH confirms that it is imminent. This dual-hormone tracking method allows for more precise timing. For instance, fertility monitors signal “High Fertility” when E3G levels rise and “Peak Fertility” when the LH surge is detected.
E3G During Pregnancy
After an embryo implants in the uterine wall, the role of E3G shifts. Throughout pregnancy, the placenta takes over as the primary producer of hormones, generating large quantities of estriol, another form of estrogen. This leads to a steady increase in E3G levels in the mother’s urine as gestation progresses. This continuous rise serves as an indicator of placental health and fetal well-being, so healthcare providers may monitor it as part of prenatal care. Consistently rising levels suggest the placenta is functioning properly, while a sudden drop could prompt further investigation.