Surge — OPK Tracker
Log your ovulation tests, watch your LH surge build, and catch your most fertile days.
Surge is Babysential's free OPK tracker. Log each ovulation predictor kit result — Negative, Low, High, or Peak — across your cycle, and the tool plots your luteinizing hormone (LH) on a simple trend chart so the surge is easy to spot. The moment you log a positive (peak), Surge tells you your fertile window is open and estimates when ovulation will follow.
According to Mayo Clinic, the LH surge precedes ovulation by roughly a day to a day and a half, which makes a positive OPK one of the clearest at-home signals that your fertile window is now. Your logs stay on your device — nothing is uploaded.
New to fertility tracking? Pair Surge with our ovulation calculator to predict your fertile days from cycle length, then switch to the due date calculator once you get a positive pregnancy test.
Understanding the LH surge
Luteinizing hormone stays low for most of your cycle, then spikes sharply just before ovulation. Ovulation predictor kits are designed to catch that spike. Because the surge can be brief — sometimes detectable for less than a day — testing consistently around the same time each day, starting a few days before you expect to ovulate, gives you the best chance of catching the true peak. According to ACOG, combining ovulation tests with calendar tracking improves your read on your fertile window.
Tips for accurate OPK testing
- Test at a consistent time: LH is often easiest to detect in the late morning or early afternoon, not first-morning urine.
- Limit fluids beforehand: Drinking a lot of water in the hours before a test can dilute your urine and mask the surge.
- Watch the line progression: Log High readings so you don't miss the jump to a true Peak — the surge can move fast.
- Combine signals: Cervical mucus changes and a basal body temperature rise after ovulation can confirm what your OPK suggests.
Frequently Asked Questions
What does an ovulation predictor kit (OPK) actually detect?▾
An OPK detects luteinizing hormone (LH) in your urine. LH rises sharply 24–36 hours before ovulation — this is called the LH surge. A positive (peak) test means your body is about to release an egg, so the LH surge is one of the most reliable at-home signals of imminent ovulation.
When should I start testing each cycle?▾
For a typical 28-day cycle, most people begin testing around cycle day 10–11. If your cycles are shorter or longer, subtract about 17 days from your usual cycle length to find a good starting day. Test once or twice daily — many find late morning to early afternoon works well — and keep testing until you see a clear positive (peak).
What does the line progression mean on an OPK?▾
A test line that is lighter than the control line is negative or low. As LH rises, the test line darkens (high). When the test line is as dark as or darker than the control line, that is a positive — your surge. Digital and quantitative tests report this as a high/peak symbol or a test-to-control ratio of roughly 1.0 or higher.
When am I most fertile after a positive OPK?▾
A positive (peak) OPK means ovulation is likely within 12–36 hours. Your highest-probability conception days are the day of the surge and the following 1–2 days. Because sperm can survive several days, the days just before the surge also count toward your fertile window.
How accurate are ovulation tests?▾
OPKs are highly reliable at detecting the LH surge, but a surge does not guarantee ovulation in every cycle, and conditions like PCOS can cause misleading readings. Combine OPKs with other signs (basal body temperature, cervical mucus) for a fuller picture, and talk to your healthcare provider if results are confusing or you have been trying for a while.