Most home pregnancy tests can detect a pregnancy about 10 days after conception, but accuracy improves dramatically the closer you get to your missed period. The timing depends on how quickly your body produces the pregnancy hormone hCG after a fertilized egg implants in your uterus, and how sensitive the test you’re using is.
What Happens in Your Body Before a Test Can Work
After an egg is fertilized, it takes about six days to travel down and implant into the lining of your uterus. Only after implantation does your body start producing hCG, the hormone pregnancy tests are designed to detect. This means there’s a built-in delay of roughly a week between conception and the point where any test could possibly pick up a signal.
Once implantation happens, hCG levels start low and rise fast, doubling approximately every 72 hours. A level below 5 mIU/mL is considered negative. Anything above 25 mIU/mL is considered a confirmed positive. The tricky zone is between 6 and 24 mIU/mL, where results are inconclusive and retesting a couple of days later is the only way to know for sure. How quickly your hCG crosses the detection threshold of your specific test determines whether you’ll see a positive result on any given day.
How Early Detection Tests Actually Perform
Some tests are marketed for use up to six days before your missed period, and they can work that early, but the odds aren’t great. FDA testing data for one of the most sensitive early-detection tests on the market shows how dramatically accuracy shifts day by day:
- 5 days before expected period: 68% of pregnancies detected
- 4 days before: 89% detected
- 3 days before: 100% detected
- 2 days before: 100% detected
- 1 day before: 100% detected
That 68% at five days out means roughly one in three pregnant people will get a false negative if they test that early. The same test reaches near-perfect accuracy just two days later. This is purely a matter of hCG concentration. At very low levels (around 3 mIU/mL), only about 5% of consumers reading these tests got a positive result. At 12 mIU/mL, 100% read it correctly. Your body simply may not have produced enough hormone yet in those first few days.
The “99% accurate” claim you see on packaging refers to performance on the day of your missed period, when hCG levels are high enough for reliable detection. It does not mean 99% accurate at any point before that.
The Best Day to Take a Test
For the most reliable result, test on the day of your expected period or later. If your cycle is regular, that’s typically about 14 days after ovulation. At this point, even standard-sensitivity tests perform well because hCG has had enough time to build up.
If you can’t wait that long, testing three days before your expected period gives you strong odds with an early-detection test. Testing earlier than that is possible but comes with a real chance of a negative result that doesn’t mean you aren’t pregnant. If you get a negative and your period still doesn’t come, test again in two to three days. The rapid doubling of hCG means a test that was negative on Monday could easily be positive by Wednesday or Thursday.
Blood Tests Detect Pregnancy Sooner
Blood tests ordered by a doctor can detect hCG within 7 to 10 days after conception, slightly earlier than home urine tests. They measure the exact amount of hCG in your blood rather than just indicating whether it crosses a threshold, which makes them more sensitive at very low levels. A doctor might order one if you’re undergoing fertility treatment, have a history of ectopic pregnancy, or need confirmation before a urine test would be reliable.
Why Timing of Day Matters
Most test instructions tell you to use your first morning urine, and there’s a good reason: your urine is more concentrated after a night of not drinking fluids, which means more hCG per sample. If you drink a lot of water before testing, you dilute the hCG in your urine, which can push it below the test’s detection threshold, especially in the early days when levels are still low. This is less of a concern once you’re past your missed period, when hCG is typically high enough that dilution won’t matter.
Reading Results at the Right Time
Every test has a reaction window, usually two to five minutes depending on the brand. Reading the result within that window is important. If you check the test too late (say, coming back to look at it an hour later), urine can leave a faint mark on the test strip as it dries. This is called an evaporation line, and it can look like a faint positive even when no hCG was detected. A true positive will appear within the time frame listed in the instructions, with a clearly visible line or symbol. If you’re unsure whether what you’re seeing is a real result or an evaporation line, take a fresh test and read it on time.
False Negatives vs. False Positives
False negatives are far more common than false positives, and almost always happen because you tested too early. Your body simply hasn’t produced enough hCG yet. Other causes include diluted urine from heavy fluid intake or a test that’s past its expiration date.
False positives are rare but can occur. Certain fertility medications contain hCG and will trigger a positive result. A very early pregnancy loss (sometimes called a chemical pregnancy) can also produce a brief positive followed by a negative and the arrival of your period. In extremely rare cases very far into pregnancy, hCG levels can climb so high (around 500,000 mIU/mL) that they overwhelm the test’s chemistry and produce a false negative, a phenomenon called the hook effect. This only applies to people who are already many weeks pregnant, not to someone testing early.
The simplest way to trust your result: if you get a positive, it’s almost certainly real. If you get a negative but suspect you might be pregnant, wait two to three days and test again with first morning urine.