LH comes from the same pituitary gland as FSH. Through most of the cycle it stays low, then rises sharply mid-cycle — the "LH surge" — which triggers the dominant follicle to release its egg roughly 24–36 hours later.
Why it matters when trying to conceive
Home ovulation predictor kits work by detecting this LH surge in urine. A positive kit means ovulation is likely within the next day or so — which is why the day of the surge and the day after are key days in the fertile window.
In fertility treatment
LH levels are checked in blood tests during cycle monitoring, and timed IUI or intercourse is often planned around the surge. In IVF, a different medicine (a "trigger injection") is used to mimic the LH surge and mature the eggs at exactly the right time before egg retrieval.