Luteinizing Hormone (LH) is a protein hormone secreted by the pituitary gland that regulates the reproductive cycle. During the follicular phase, LH works alongside Follicle-Stimulating Hormone (FSH) to help mature an egg within an ovarian follicle. A dramatic, temporary rise in LH concentration, known as the LH surge, is the biological signal that triggers the final steps of this process. This surge leads directly to the release of the mature egg, making the timing of this hormonal event fundamental for fertility tracking.
The Purpose and Timing of the LH Surge
The LH surge is initiated by a significant increase in estrogen, specifically estradiol, produced by the developing ovarian follicle. As the follicle nears maturity, its estrogen output reaches a high threshold that signals the pituitary gland to release a large, rapid burst of LH. This positive feedback loop reverses the previous low-level regulation of hormones. The surge in LH is the biological trigger that causes the dominant follicle to rupture and release the egg.
This hormonal event begins approximately 36 hours before the egg is released. The entire surge typically lasts between 12 and 36 hours, though this duration can vary between individuals and cycles. The purpose of this sudden spike is to prompt the final maturation of the egg and prepare the ovarian wall for ovulation.
The Peak and Synchronization with Ovulation
The LH peak represents the moment when the luteinizing hormone concentration reaches its highest point during the entire surge. This peak concentration is not the moment of ovulation itself, but rather the immediate precursor. The maximum LH concentration precedes the release of the egg by a short, defined window.
Ovulation, the release of the mature egg, follows the LH peak by an estimated 10 to 12 hours. The egg is released between 24 and 36 hours after the start of the LH surge is first detected. This timing makes detecting the surge a reliable predictor of the most fertile window. The egg is viable for fertilization for only about 12 to 24 hours, which emphasizes the value of accurately predicting the peak.
Interpreting the Post-Peak Drop
LH levels drop on ovulation day because the egg is released after the peak of the surge. Since ovulation occurs 10 to 12 hours following the maximum LH concentration, the hormone level has already begun its steep decline toward baseline when the egg is released. The LH surge is a transient event designed only to trigger the rupture of the follicle.
Once the egg is released, the ruptured follicle transforms into the corpus luteum, a temporary gland that produces large amounts of progesterone. This shift in hormonal control marks the beginning of the luteal phase, and high concentrations of LH are no longer needed. Within 24 to 48 hours after ovulation, LH levels rapidly return to the low concentrations characteristic of the luteal phase. Therefore, on the day of ovulation, LH levels are falling sharply, not peaking.
Practical Application: Tracking LH with Ovulation Predictor Kits
Ovulation Predictor Kits (OPKs) are designed to detect the presence of the LH surge in the urine. These kits use test strips that compare the intensity of a test line to a control line. A positive result occurs when the test line is as dark as or darker than the control line, signifying the LH surge is underway. This means ovulation is expected in the next 12 to 36 hours, placing the user in their most fertile window.
Testing once or twice a day during the predicted fertile window is recommended to avoid missing the short-lived surge. Once a positive result is obtained, there is no need to continue testing for that cycle, as the test’s purpose is predictive. The visual confirmation of the LH level fading back to a faint line on subsequent tests confirms the hormone has dropped and the fertile window has passed.