T/C ratio on ovulation tests: how to read it and when to have sex

What T/C ratio means, which result counts as positive, when to start testing, and when to have sex.

T/C ratio compares how dark the test line (T) on an ovulation test is with the control line (C). A T/C ratio of 1.0 or higher, meaning the test line is as dark as or darker than the control line, is a positive result: your LH surge has started, and ovulation usually follows within 24 to 36 hours. A ratio below 1.0 is negative, even if you can see a faint test line.

Your best chance of pregnancy is to have sex on the day of your first positive and the day after. This guide shows how to read the T/C ratio, when to start testing, and how to time sex so you can stop second-guessing faint lines.

Your fertile window is the only time each cycle when pregnancy can happen, and it is shorter than most people think. It includes:

• 5 days before ovulation: sperm can survive up to 5 days in fertile cervical mucus.
• The day of ovulation itself: the egg lives for only 12 to 24 hours after release.

Why timing matters: missing this window means waiting another full cycle. Sperm need time to travel and get ready to fertilize the egg, so having sex before the egg is released works better than waiting until ovulation day. ASRM guidance shows pregnancy rates are highest when sex happens in the 2 days before ovulation.

Chance of pregnancy from sex on each day. It is highest in the 2 to 3 days up to ovulation.

Based on Wilcox et al., 1995. Timing of sexual intercourse in relation to ovulation.

Ovulation predictor kits (OPKs) work by detecting luteinizing hormone (LH) in your urine. Here is how it works:

• LH triggers ovulation: 24 to 36 hours before your ovary releases an egg, your pituitary gland releases a surge of LH.
• The surge is detectable: LH levels spike from a low baseline to a much higher peak for about a day.
• Urine reflects blood levels: what is in your bloodstream shows up in your urine within hours.

The T/C ratio (test line compared with control line) is the standard way to read an ovulation test strip. A positive result means T/C is 1.0 or higher: the test line is as dark as or darker than the control line. This means your LH has risen enough to signal that ovulation is coming.

Best for most women: strip tests offer the best value for frequent testing and let you track LH as it rises, which helps you learn your own pattern.

Generic advice usually says "start on day 10," but this often leads to wasted tests or missed surges.

Ovulation typically happens 14 days before your next period, so the best time to start testing is 17 days before your next expected period. This gives you a 3-day buffer to catch the LH rise before it peaks.

1. Count the days from the first day of your last period to the first day of your next period. This is your cycle length.
2. Subtract 17 from that number.
3. Start testing on that cycle day.

Example: 32-day cycle → 32 − 17 = day 15 (start testing on day 15).

For a full chart by cycle length, see when to start testing.

Use your shortest cycle from the last few months in the formula (shortest cycle minus 17), and test daily until you get a positive or your period starts. Yes, you will use more tests, but missing ovulation costs you a whole month.

When cycles vary by 7 or more days: consider testing twice a day (late morning and evening) during your estimated fertile window to catch short LH surges that last less than 24 hours.

Stop second-guessing faint lines. Here is the objective standard.

• Negative (not yet fertile): the test line is lighter than the control line (T/C ratio below 1.0). LH is present but has not surged yet. Action: keep testing daily.
• Positive (peak fertility): the test line is as dark as or darker than the control line (T/C ratio 1.0 or higher). LH surge detected. Action: have sex today and tomorrow.

The T/C ratio turns a judgment call into a number. Instead of asking whether a line looks dark enough, you compare T with C. When you scan a strip with the Pregmate app, it shows the T/C ratio as a number, so you can watch it rise across your cycle.

Based on Pregmate test instructions.

• Empty circle or blank: negative
• Smiley face or "Yes": positive (LH surge detected)
• Flashing smiley (advanced tests): high estrogen detected, fertility rising but not yet at peak

Between late morning and evening (about 10 AM to 8 PM) is ideal. Here is why:

• LH surges often begin in the early morning.
• First morning urine may miss the surge because it collected overnight, before the surge started.
• Late morning to evening urine reflects your current LH level.

Testing twice a day: if you have short cycles or unpredictable ovulation, test at about 10 AM and 6 PM. The surge can be as brief as 12 to 14 hours, so twice-daily testing helps you catch it.

• Don't drink large amounts of water in the 2 hours before testing. Diluted urine can cause false negatives.
• Normal hydration is fine.
• If your urine is very pale or clear, wait an hour and test again.

Try not to pee for about 2 hours before testing. This lets LH build up in your urine so the test can detect it.

• Test between late morning and evening (not first morning urine).
• Hold urine for about 2 hours before testing.
• Avoid drinking a lot of water in the 2 hours before.
• Dip the strip up to the MAX line for 5 seconds (follow package instructions).
• Read the result at 5 minutes. Don't read it later.
• Scan the strip in the Pregmate app or write the result down.
• If positive: have sex today and tomorrow.

Ovulation tests warn you before ovulation. Other body signs help fill in the picture.

1. Basal body temperature (BBT)

• What it measures: your resting body temperature rises about 0.5 to 1°F after ovulation because of progesterone.
• How to use it: take your temperature first thing every morning, before getting out of bed.
• Why combine: a lasting BBT rise suggests ovulation has already happened, while ovulation tests warn you before it happens. Together, they show your pattern over a few cycles.

2. Cervical mucus observation

• Fertile mucus looks like: clear, stretchy, slippery (like raw egg whites).
• Timing: appears a few days before ovulation and is usually most abundant in the 1 to 2 days before it.
• Why combine: it gives you an early warning. Start testing more often when you see fertile mucus.

3. Ovulation pain (mittelschmerz)

• What it feels like: a one-sided twinge or ache in the lower abdomen that lasts from a few hours to a day.
• When it occurs: around ovulation.
• Why combine: it can be a helpful extra sign, but not everyone feels it, so don't rely on it alone.

Once you get a positive ovulation test:

1. Have sex on the day of your first positive test.
2. Have sex again the next day.
3. Before your positive, sex every 1 to 2 days covers the rest of your fertile window.

The day of your positive and the day after are your two most fertile days.

The day of your positive and the day after are your two most fertile days, because ovulation usually follows the surge by 1 to 1½ days. Sperm that are already waiting when the egg is released give you the best chance.

No. According to ASRM guidance, sex more often than every 1 to 2 days doesn't lower your chances of getting pregnant. Choose the pace that feels comfortable for you both.

1. Testing too late in your cycle

• The problem: you start testing on day 14, but you actually ovulate on day 11.
• The fix: calculate your start day with the formula above. If in doubt, start earlier.

2. Short LH surge

• The problem: your LH surge lasts only 10 to 12 hours, and you're testing once a day.
• The fix: test twice a day (about 8 hours apart) during your expected fertile window.

3. Anovulatory cycles

• The problem: in some cycles, you don't ovulate at all. This is more common with PMOS (formerly PCOS), high stress, or perimenopause. See ovulation tests with PMOS.
• The fix: track for 3 months. If you never see a positive, ask a doctor about hormone testing.

4. Low LH surge

• The problem: your surge doesn't reach the test's detection threshold.
• The fix: watch the T/C ratio, not just the line color. A ratio that rises toward 0.8–1.0 and then drops may still be your surge. A fertility monitor that also tracks estrogen can help too.

5. Testing window missed

• The problem: irregular cycles make it hard to predict when to test.
• The fix: start earlier, using your shortest cycle in the formula, and continue for 14 or more days if needed.

Talk to a doctor if you've tracked 3 or more cycles with no positive test, or if you haven't gotten pregnant after 12 months of trying (under 35) or 6 months (35 or older). If you're 40 or older or your cycles are irregular, talk to a doctor now. Blood tests can check LH, FSH, AMH (ovarian reserve), and progesterone.

• Practice Committee of the American Society for Reproductive Medicine. Optimizing natural fertility: a committee opinion. Fertil Steril. 2022;117(1):53-63. ASRM
• Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med. 1995;333(23):1517-1521. NEJM
• Ecochard R, Duterque O, Leiva R, Bouchard T, Vigil P. Self-identification of the clinical fertile window and the ovulation period. Fertil Steril. 2015;103(5):1319-1325. Fertility and Sterility
• McGovern PG, Myers ER, Silva S, et al. Absence of secretory endometrium after false-positive home urine luteinizing hormone testing. Fertil Steril. 2004;82(5):1273-1277. PubMed
• Johnson S, Weddell S, Godbert S, Freundl G, Roos J, Gnoth C. Development of the first urinary reproductive hormone ranges referenced to independently determined ovulation day. Clin Chem Lab Med. 2015;53(7):1099-1109. De Gruyter
• U.S. Food and Drug Administration. Ovulation (urine test). FDA
• Mayo Clinic Press. Finding your fertility window. Mayo Clinic Press
• Cleveland Clinic. Ovulation: calculating, timeline, pain and other symptoms. Cleveland Clinic
• Pregmate. Ovulation test strips: instructions for use. Pregmate