Your body gives you a 24–48 hour warning before ovulation. Here's how to detect it and use it to time conception perfectly.
LH stands for luteinising hormone — a hormone released by the pituitary gland that triggers the release of a mature egg from the ovary. The LH surge is a sudden, sharp spike in LH levels that occurs roughly 24–48 hours before ovulation. Detecting this surge with an ovulation predictor kit (OPK) or through fertility apps gives you advance warning of your most fertile days.
As follicles in your ovaries develop, estrogen builds steadily. Your cervical mucus becomes wetter and more egg-white-like.
Once estrogen crosses a threshold, it triggers a massive surge in LH — rising 5–10× above baseline within hours. OPKs turn positive.
LH triggers the dominant follicle to rupture, releasing the egg into the fallopian tube. You may notice a sharp twinge of pain (mittelschmerz) on one side.
The empty follicle becomes the corpus luteum, producing progesterone. LH drops back to baseline. OPKs become negative again.
OPKs detect LH in your urine. A positive result (test line as dark or darker than the control line) indicates your LH has surged. Key tips for accurate results:
In a textbook 28-day cycle, the LH surge occurs around days 12–14. But cycles vary. The more reliable rule is: the LH surge occurs about 14 days before your next period, regardless of cycle length. So if your cycle is 35 days, your surge is likely around day 21. If your cycle is 24 days, it's around day 10.
Excellent timing. Sperm deposited now will be waiting when the egg is released.
Best day. Ovulation most likely occurring within this 24-hour window.
Still possible if ovulation was slightly delayed, but fertility is declining.
The egg has likely passed. Fertile window has closed for this cycle.
If you're testing consistently and not detecting a surge, possible explanations include: testing at the wrong time of day, cycles where ovulation isn't occurring (anovulatory cycles — common with PCOS), very short surges (some women's surges last only a few hours), or tests that aren't sensitive enough.
Some women — particularly those with PCOS — may have multiple LH spikes before actual ovulation occurs. This can make OPKs confusing. Combining OPKs with basal body temperature tracking and cervical mucus monitoring gives a clearer picture.
WomensPal tracks your cycle history and predicts your fertile window and ovulation day. Combine with our ovulation calculator for even more precision.
Try the ovulation calculator →An LH (luteinising hormone) surge is a rapid spike in LH levels that triggers ovulation. It typically occurs about 24–48 hours before the egg is released and can be detected with ovulation predictor kits (OPKs).
In a 28-day cycle, the LH surge typically happens around day 12–14. In shorter or longer cycles, it occurs approximately 14 days before your next expected period.
An LH surge typically lasts 12–48 hours. Ovulation usually occurs within 24–36 hours of the surge beginning. After the egg is released, LH levels rapidly drop back to baseline.
A positive OPK means your LH has surged — ovulation is likely within 24–48 hours. This is your most fertile window. Having sex on the day of the positive and the following 1–2 days gives you the best chance of conception.
Related: Ovulation Calculator · Signs of Ovulation · Fertile Window Guide · How to Get Pregnant Fast