What the curve is showing
Luteinising hormone sits at a low baseline for most of the cycle, which is the faint line you see on the first few strips. When the dominant follicle is ready, the pituitary releases a burst of LH that rises over about a day, peaks, and falls back within one to three days. The egg is released roughly 24 to 36 hours after the surge starts. On the chart that is a flat start, a climb, a point at the top, and a drop; the ovulation marker sits one day after the first positive.
How the prediction works
Before your readings say anything, the surge is placed at cycle day (cycle length minus 14), the day before the ovulation estimate the ovulation calculator uses. Once a reading is clearly darker than the one before, the prediction moves to your data: a medium line predicts the surge in about two days, an almost-positive line predicts it tomorrow, and a positive is the surge. After a positive, a fading reading two days later marks the surge as complete. If the expected day passes with flat readings, the tool says so rather than guessing, and the LH test troubleshooter takes over.
Using it well
Test between 10am and 8pm, hold drinks for two hours, and read the strip within the time on the packet. Use the same brand through the cycle so darkness is comparable. Add a second reading on any day the line jumps; the tool keeps one reading per day, so record the darker of the two. When the chart shows the surge, the best time to get pregnant calculator lays out the days around it, and once you are past ovulation the how many DPO am I page counts down to a test.
Frequently asked questions
How long after the LH surge do you ovulate?
Usually 24 to 36 hours after the surge begins, which in practice is the day after your first positive test. Because urine lags blood by a few hours and most people test once a day, the first positive strip often catches the surge some hours in, so ovulation can be as soon as the next morning.
What does a rising LH line mean?
A test line that is clearly darker than the day before means LH is climbing towards the surge. Start trying now, because sperm can wait up to five days for the egg, and switch to testing twice a day so the peak itself is not missed.
Is a faint line on an ovulation test positive?
No. LH is always present at a low level, so a faint line is the normal baseline and counts as negative. Only a test line as dark as, or darker than, the control line is positive.
What LH level is a positive ovulation test?
Most strips turn positive at about 25 mIU/mL, some at 30 or 40. Baseline LH is typically 2 to 10 mIU/mL and the surge peaks anywhere from 25 to over 100. If your reader gives a number, enter it in mIU/mL mode with your test's threshold.
Why is my LH surge on a different day from the calculator's estimate?
The estimate assumes ovulation 14 days before the next period. Real cycles vary by several days, especially after illness, travel, stress or a change in sleep, and the follicular phase is the part that varies most. Your readings override the estimate as soon as the line starts to rise.
What if the line never gets as dark as the control?
Test in the afternoon, hold fluids for two hours, and check you started early enough. Some people, especially with PCOS, have a raised baseline that blurs the surge. The LH test troubleshooter works through the possibilities, and basal body temperature or a progesterone blood test confirms ovulation another way.
Sources
- Luteinizing hormone (LH) levels test - MedlinePlus, US National Library of Medicine.
- Fertility problems: assessment and treatment (NG257) - NICE, 2026.
- Timing of sexual intercourse in relation to ovulation - New England Journal of Medicine (Wilcox et al.), 1995.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.