# When to Take a Pregnancy Test _Test from the first day of your missed period; a week later is the most reliable. Find both days for your cycle._ - Source: https://proba.baby/en/tools/pregnancy-test-calculator - Language: English - Last verified: v1.0.4 · 7 Sep 2026 - Last updated: 2026-10-11 ## How it works The short answer: test on the day your period is due, the first day of a missed period. That is the earliest day most home tests are made for. If the test is negative and your period still has not come, test again a week later, when a test is most reliable. If you do not know when your period is due, the NHS says to test at least 21 days after you last had unprotected sex. Work it out from the first day of your last period and your cycle length, or from your ovulation day if you know it from an ovulation test or a temperature chart. You get both test days, the implantation window before them and a short note on what to do today: wait, test now, or test now for the most reliable result. Why wait at all? Tests look for the hormone hCG, which the body only makes after implantation, 6 to 12 days after ovulation. On the first day of a missed period about 1 pregnancy in 10 has not implanted yet (Wilcox 2001), so testing earlier mostly buys a negative result you cannot trust. This page gives dates only: it does not read test results, it is not a diagnosis and it is not contraception. ## The math From your last period: your period is due on the first day of your last period + cycle length, the ProbaBaby app's Planning rule. From a known ovulation day: ovulation + 15 days, an average 14-day second half of the cycle. Earliest test day = the day your period is due. Most reliable test day = 7 days later. These match the app's own test timing and Wilcox 2001: with a highly sensitive laboratory test, at most about 90% of pregnancies were detectable on the first day of the missed period and about 97% a week later. The implantation window shown with them is 6 to 12 days after ovulation (Wilcox 1999), with ovulation estimated at cycle length minus 14 days. ## FAQ **How soon can I take a pregnancy test?** From the first day of your missed period for most tests. Some very sensitive tests are sold for use a few days earlier, but even a perfect test cannot find a pregnancy before implantation, and about 1 in 10 pregnancies has not implanted by the day a period is due (Wilcox 2001). **How many days after sex should I test?** If you know when your period is due, test on that day. If you do not, the NHS says to test at least 21 days after you last had unprotected sex. Ovulation, fertilisation and implantation all have to happen first, and hCG then needs a few days to rise. **Is morning urine better?** For most tests it does not matter. The NHS says you can use urine collected at any time of day. Follow the instructions of your test, and avoid drinking a lot of water just before testing. **My test was negative but my period has not come. What now?** Wait a few days and test again, or test on the most reliable day this page shows. If a second test is negative and your period still has not come, see your doctor (NHS). Stress, illness, weight change and some contraception can all delay a period. **Can this tell me if I am pregnant?** No. It only tells you when a test is worth taking. It does not read test results and it cannot tell you anything about a pregnancy. If your test is positive, contact your doctor or midwife to start your care. **When should I get help straight away?** Severe or one-sided pain low in your belly, especially with a positive test or a late period, needs help the same day. Very heavy bleeding, or bleeding with severe pain, dizziness or fainting, needs your emergency number now. ## Sources - Wilcox AJ, Baird DD, Dunson D, McChesney R, Weinberg CR. Natural limits of pregnancy testing in relation to the expected menstrual period. JAMA 2001;286:1759-61: https://europepmc.org/article/MED/11594902 - NHS: Doing a pregnancy test: https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/ - Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med 1999;340:1796-9: https://europepmc.org/article/MED/10362823 - NHS: Missed or late periods: https://www.nhs.uk/conditions/stopped-or-missed-periods/ - NHS: Signs and symptoms of pregnancy: https://www.nhs.uk/pregnancy/trying-for-a-baby/signs-and-symptoms-of-pregnancy/ --- ProbaBaby (listed on the App Store as “Pregnancy & Baby Tracker - PB”) is a free iPhone, iPad and Apple Watch app for pregnancy, newborn and postpartum tracking, built for twins, in 32 in-app languages, with an optional AI baby-face prediction that computes Mendelian genetics (Punnett squares) from the traits you enter before any image is generated. Tracking, tools, PDF reports and family sharing are free; only AI predictions use credits — 5 starter credits cover your first Scientific prediction — and no subscription is required. Requires iOS 26 or later. Not available on Android.