For the most accurate result, take a home pregnancy test after the first day of your missed period. Testing earlier is possible, but the hormone that pregnancy tests detect may not be at high enough levels yet. Most home tests claim 99% accuracy, but that number depends heavily on timing.
Why Timing Matters
Home pregnancy tests work by detecting a hormone called hCG in your urine. Your body only starts producing hCG after a fertilized egg implants in the uterine wall, which happens between six and ten days after ovulation. Even once implantation occurs, hCG levels start extremely low and roughly double every two to three days. A test taken too early simply won’t find enough of the hormone to register a positive result.
For most urine tests, hCG becomes detectable about 10 days after conception. That lines up closely with the first day of a missed period for people with a regular 28-day cycle. Testing before that point increases your chance of getting a false negative, where you are pregnant but the test says you’re not.
What to Do With Irregular Cycles
If your period doesn’t arrive on a predictable schedule, pinpointing “the first day of a missed period” isn’t straightforward. The Office on Women’s Health recommends counting 36 days from the start of your last menstrual cycle, or four weeks from the time you had sex. By that point, hCG levels should be high enough for a home test to pick up.
How to Get the Most Accurate Result
Beyond choosing the right day, a few practical steps improve reliability:
- Use first morning urine. Your urine is most concentrated after a full night without drinking fluids, so it contains the highest level of hCG. Testing later in the day, especially after drinking a lot of water, can dilute the hormone enough that the test misses it.
- Don’t over-hydrate beforehand. Drinking large amounts of fluid before collecting your sample lowers hCG concentration and can produce a false negative.
- Read results within the time window. Every test brand specifies a reaction time, usually a few minutes. Reading the test after that window (past about 10 minutes) can cause a faint, colorless streak to appear as the urine dries. This is called an evaporation line, and it is not a positive result.
Faint Lines and Evaporation Lines
A faint colored line on a pregnancy test is generally a positive result. Early in pregnancy, when hCG is still building, the test line can look lighter or slightly blurred compared to the control line. That’s normal. What matters is that the line has actual color matching the control line.
An evaporation line is different. It’s a colorless streak that shows up after the test dries, well past the recommended reading window. If you see one clear colored line and a second faint, colorless line, that’s evaporation, not a positive. When in doubt, take a fresh test the next morning and read it within the manufacturer’s time frame.
If Your Test Is Negative but Your Period Doesn’t Come
A negative result doesn’t always mean you aren’t pregnant. You may have ovulated later than usual, which pushes the entire timeline back. The Mayo Clinic recommends retesting one week after your missed period if you still suspect pregnancy. By that point, hCG levels in a pregnant person are typically high enough for any home test to detect.
If the second test is also negative and your period still hasn’t arrived, other factors like stress, weight changes, or hormonal shifts could be affecting your cycle.
Blood Tests vs. Home Urine Tests
A blood test ordered through a doctor’s office can detect pregnancy slightly earlier than a home urine test. Blood tests pick up very small amounts of hCG, making them reliable as early as seven to ten days after conception. That’s potentially a few days sooner than a urine test. They’re not routine for most people, but they’re useful if you need an early answer, if home results are unclear, or if your doctor wants to track how quickly hCG levels are rising.
Medications That Can Affect Results
Certain medications interfere with pregnancy test accuracy. The most common culprits are fertility treatments that contain hCG directly, since the test can’t distinguish between the medication and pregnancy-produced hCG. If you’ve recently had an hCG injection as part of fertility treatment, your doctor can advise on how long to wait before testing.
A few other categories of medication can occasionally cause false positives: some antipsychotic medications, certain anti-seizure drugs, anti-nausea medications, and some progestin-only birth control pills. If you take any of these and get an unexpected positive, a blood test through your doctor can confirm the result.