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How can an LH surge in urine indicate that ovulation has occurred?

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Diagram of the menstrual cycle showing hormone levels, follicle development, and endometrial changes across 28 days. It highlights menstruation, ovulation, and the follicular and luteal phases, with labeled curves for estrogen, progesterone, FSH, and LH. — menstruation Cyclical changes during a woman's normal ovulatory menstrual cycle.

During the menstrual cycle, the maturation of ovarian follicles leads to an increase in estradiol secretion, which triggers a transient surge in luteinizing hormone (LH). This surge is the physiological mechanism that causes the mature Graafian follicle to rupture, resulting in ovulation.12

Because this preovulatory elevation of LH is detectable in urine, women can use sensitive home test kits to identify the surge. Detecting this spike provides a practical method to determine that the hormonal processes leading to ovulation are underway, indicating that a woman is in her potentially fertile window. While the LH surge precedes the release of the egg, it serves as a reliable clinical and personal indicator that ovulation is imminent or has been triggered.31