Luteinizing hormone (LH) is a protein hormone produced by the pituitary gland. This hormone signals the ovary to release a mature egg, a process known as ovulation. Throughout the follicular phase, LH levels remain relatively low, but a significant, rapid increase is the definitive marker that ovulation is imminent. Tracking this hormonal rise is the basis for at-home ovulation prediction tests used by many people seeking to time conception.
The Role of Luteinizing Hormone in the Menstrual Cycle
Luteinizing hormone is one of the two gonadotropins, alongside follicle-stimulating hormone (FSH), released from the anterior pituitary gland. During the follicular phase, LH works with FSH to stimulate ovarian follicles to grow and mature. LH specifically supports specialized cells in the ovary that produce androgen precursors, which are then converted into estradiol.
The developing follicles gradually secrete increasing amounts of estradiol, which initially suppresses the production of LH through a negative feedback loop. This initial suppression helps ensure only one follicle becomes dominant and prepared for release. As the cycle progresses, the dominant follicle reaches a size where its estradiol production crosses a certain concentration threshold.
This sustained, high level of estradiol then causes a paradoxical switch in the brain’s signaling pathway. Instead of suppressing the pituitary, the high estradiol concentration begins to stimulate it, triggering a massive release of LH. This event is known as the LH surge, and it is the direct signal for the ovary to complete the final stages of egg maturation and release. The entire surge typically lasts between 24 and 48 hours.
The Immediate Pre-Ovulatory Pattern
LH levels are not perfectly steady during the follicular phase, and minor fluctuations or pulses occur before the main surge. These small increases may sometimes be interpreted as “mini-surges” when tracking with daily tests, but they do not lead to ovulation.
The transition to the main surge is governed by the sustained rise of estradiol, not by minor LH variations. Once estradiol reaches its required peak, it switches the hypothalamic-pituitary-ovarian axis from a negative feedback system to a positive one. This positive feedback causes the rapid rise in LH, which is distinct from any earlier shifts.
Scientific studies suggest that less than half of people experience the textbook, single, sharp LH surge pattern. Many people (up to 44%) have a pattern involving an initial rise, a small dip, and then a second, true surge. Another pattern (about 15% of cycles) is a “plateau,” where LH levels rise and remain high for a longer duration. The first significantly elevated LH reading marks the beginning of the ovulatory process, regardless of these variations.
Interpreting the LH Surge: Timing Ovulation
Tracking LH identifies the start of the true surge, which pinpoints the most fertile days. Ovulation prediction kits (OPKs) detect this rise by measuring LH concentration in urine against a baseline. The first test showing a significant increase above this baseline signals the opening of the fertile window.
Ovulation does not happen at the exact moment the test turns positive. It typically occurs between 24 and 36 hours after the LH surge begins, though some studies suggest this window can be up to 48 hours. The egg is only viable for approximately 12 to 24 hours after its release, making the period immediately following the surge the most fertile time.
The most important reading is the initial, definitive rise in LH, rather than tracking minor pre-surge fluctuations. Once the test detects a positive surge, the window for egg release is open, and the timing of intercourse can be planned accordingly.