LH surge

What is the LH surge?

Luteinizing hormone (LH) is made by the pituitary gland and stays at a low background level for most of your cycle. As an egg in the ovary finishes maturing, rising estrogen sends a signal back to the brain that flips a switch: the pituitary releases a sudden burst of LH. That burst is the “LH surge.” It is the trigger that prompts the mature follicle to release its egg — the event we call ovulation.

Think of the surge as the starting gun rather than the finish line. The level of LH climbs quickly, peaks, and then drops back down, all within a day or two. That brief peak is exactly what makes the surge so useful for timing: it is a reliable, hormone-based marker that ovulation is about to happen, and it shows up just before your most fertile window opens.

The LH surge triggers ovulation ~24–36 hours later

Once the surge begins, ovulation typically follows around 24 to 36 hours afterward. The exact timing varies from person to person and even cycle to cycle, but the pattern is consistent enough to plan around. This is why catching the surge matters so much for conception: it gives you roughly a one-to-two-day heads-up that the egg is on its way.

Pairing the surge with the lifespan of sperm and egg explains the whole fertile window. Sperm can survive several days inside the reproductive tract, while a released egg lives only about 12 to 24 hours. So the days leading up to and including ovulation are when conception is possible — and the LH surge is the natural alarm clock that tells you that window is opening.

How ovulation predictor kits (OPKs) detect it

Ovulation predictor kits are simple urine tests that measure LH. Most look like a pregnancy test — you either dip a strip or hold it in your urine stream, then read the result after a few minutes. The kit is calibrated to a threshold: when your LH crosses that level, the test reads positive, signalling that your surge has begun.

• Line tests show a control line plus a test line. The test line is read as positive only when it is as dark as, or darker than, the control line — a faint line is not a positive.
• Digital tests remove the guesswork by displaying a clear symbol, such as a smiley face, when LH crosses the threshold.
• Timing your testing matters. Start testing a few days before you expect to ovulate, test at the same time each day (late morning or early afternoon often works well), and avoid drinking large amounts of fluid beforehand so your urine is not overly diluted.

Because the surge is short, daily — or even twice-daily — testing as you approach your window gives you the best chance of catching that first positive. If you are not sure when to start, our Fertile Window Calculator estimates the days to focus on based on your cycle.

How long does the surge last?

The surge is fast. LH usually rises, peaks, and falls within roughly 24 to 48 hours, which means an OPK may only read positive for a day or two. For some people the peak is sharp and brief enough that a once-a-day test can miss it entirely — especially if the surge starts and fades overnight. That is the main reason kits recommend testing consistently rather than just once.

A few practical habits help you catch it: begin a few days earlier than you think you need to, keep your testing time consistent, and consider testing twice a day during the days you expect your window. Once you see your first clear positive, you have your signal — there is no need to keep chasing darker lines.

How to time intercourse for conception

The goal is to have sperm ready and waiting when the egg is released. Because sperm survive for several days and the egg lasts under a day, the most fertile stretch runs from a few days before ovulation through the day of ovulation itself. The LH surge marks the front edge of that stretch.

• Aim for intercourse on the day you get your positive OPK and the following day — this lines up with the ~24–36 hour gap to ovulation.
• Having sex in the days leading up to the surge helps too, since sperm can already be in place when the egg arrives.
• Every-day or every-other-day intercourse across your fertile window is a common, low-stress approach that does not rely on pinpoint timing.

Combining OPKs with other ovulation cues — such as changes in cervical mucus or a tracked rise in basal body temperature — can make your window even clearer. Our guide to the signs of ovulation walks through what to watch for alongside the surge.

Why a positive OPK means ovulation is near, not confirmed

A positive ovulation test is a forecast, not a receipt. It tells you that LH has surged and ovulation is likely soon — typically within a day or two — but it does not prove that an egg was actually released. In most cycles the surge does lead to ovulation, which is why OPKs are so useful, but the two are not guaranteed to go together every single time.

A couple of situations explain the gap. Some cycles are anovulatory, meaning a surge occurs but no egg is successfully released. And conditions such as polycystic ovary syndrome (PCOS) can keep LH elevated, producing positive tests that do not reliably line up with ovulation. So treat a positive OPK as your best signal to focus your timing — not as confirmation that ovulation has happened. If you want to know whether you actually ovulated, that usually takes additional signs or a conversation with a provider.

When to see a doctor

Ovulation tests are a helpful planning aid, not a diagnosis. It is a good idea to talk with a healthcare provider if:

• You never get a positive OPK across several cycles, or your results are confusing month after month.
• Your cycles are very irregular, very long, or you suspect you may not be ovulating.
• You have signs that can affect ovulation, such as those linked to PCOS or a thyroid imbalance.
• You have been trying to conceive for 12 months (or 6 months if you are over 35) without success.

A provider can look into what is happening, suggest tests that confirm whether ovulation is occurring, and offer reassurance or next steps — which is often all that is needed.