LH surge

The LH surge is the brief, dramatic spike in luteinizing hormone that acts as the starting gun for ovulation. For most of the cycle LH trickles out in modest pulses, but for a day or so around mid-cycle it rockets up many-fold — the single largest hormonal swing of the normal cycle.

What makes the surge remarkable is its cause: positive feedback. Throughout the follicular phase, estrogen normally suppresses LH. But once estradiol from the maturing follicle stays high enough for long enough, the hypothalamus and pituitary flip their response, so that high estrogen now stimulates rather than restrains LH release. This is one of the very few physiological examples where a hormone amplifies its own controlling signal instead of dampening it.

The surge does several things at once: it triggers the final maturation and rupture of the dominant follicle, releasing the egg, and it begins converting the leftover follicle cells into the progesterone-secreting corpus luteum (luteinization). A practical caveat: it is the rising and surging LH, detectable in urine, that ovulation predictor kits measure — they detect the trigger, not the act of ovulation itself, which follows roughly a day later.