What is the LH surge?
The LH surge is a sudden, steep rise in luteinizing hormone (LH) released by the pituitary gland in the middle of the menstrual cycle. It triggers ovulation, usually 24 to 36 hours after the surge begins. Ovulation tests detect this surge in urine, which is why a positive test means your most fertile days are now.
What causes the surge
As the leading follicle grows, it makes more and more estrogen. Once estrogen stays high for long enough, it flips from holding LH down to pushing it up. The pituitary then releases a burst of luteinizing hormone, which makes the follicle burst and release its egg.
Timing at a glance
- Surge starts: often in the early morning, roughly cycle day 12 to 14 in a 28-day cycle, later in longer cycles
- Peak: about 10 to 12 hours later
- Ovulation: about 24 to 36 hours after the surge starts
- Fertile days: the 2 days before ovulation and the day itself are the most fertile
Testing for it
An ovulation predictor kit turns positive when LH crosses a threshold. Testing once or twice a day from a few days before your expected ovulation catches most surges. Our ovulation test calculator suggests when to start.
With PCOS, LH can be high all the time, so tests may show several positives without ovulation.
Some people feel social, confident and flirty around the surge, when estrogen peaks. Others notice mittelschmerz, a twinge on one side.
Frequently asked questions
How long does an LH surge last?
Usually 24 to 48 hours, though some surges are shorter and easy to miss with once-a-day testing.
Does an LH surge always mean you ovulated?
Not always. It is a strong sign, but ovulation is only confirmed afterwards, for example by a rise in basal body temperature or a progesterone blood test.
Sources
- MedlinePlus: Luteinizing Hormone (LH) Levels Test
- MedlinePlus: Ovulation home test
- Cleveland Clinic: Ovulation
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
Luteinizing hormone (LH): definition and meaningLuteinizing hormone (LH) is a pituitary hormone that triggers ovulation and supports progesterone. Normal levels, the LH surge and what tests show.
What is an ovulation predictor kit (OPK)?An ovulation predictor kit (OPK) is a home urine test that detects the LH surge before ovulation. How OPKs work, when to test, and how to read them.
Ovulation: definition and meaningOvulation is when an ovary releases an egg, usually 12 to 14 days before your next period. Signs, timing, the fertile window, and how it affects mood.Ovulation test calculator: when to start testingFree ovulation test calculator: enter your last period and cycle length to see the day to start LH tests, plus the best time of day to test. Any brand.
Ovulation phase: signs, timing and feelingsThe ovulation phase explained: when you ovulate, how long it lasts, the signs and symptoms to look for, and why mood, energy and libido often peak.