# When should I take a pregnancy test? Works out when to take a home pregnancy test: from the first day of the missed period (from the last period or the ovulation date), or 21 days after sex, with the likely implantation window. - Page: https://www.acalculator.org/health/pregnancy-test-calculator - JSON spec: https://www.acalculator.org/health/pregnancy-test-calculator.json - Version: 7db1b8bf7a44 ## Default answer Example with the default inputs (What do you know? Last period, First day of your last period September 8, 2026, Cycle length (days) 28) on the example date Tuesday, September 29, 2026: Take a home pregnancy test on or after October 6, 2026. ## Inputs | Key | Label | Description | | --- | --- | --- | | from | What do you know? | The date you know: the first day of your last period, your ovulation day, or the day you had sex. | | lmp | First day of your last period | The first day of bleeding of your most recent period. | | cycle | Cycle length (days) | Days from the first day of one period to the first day of the next; 28 on average. | | ovulation | Ovulation day | The day you ovulated, for example from an ovulation test or temperature chart. | | sex | Day you had sex | The day of the unprotected sex you want to test for. | ## Outputs | Key | Label | Description | | --- | --- | --- | | test | Test on or after | The first day a home test is worth taking: the missed period, or 21 days after sex. | | reliable | More reliable from | One week after the missed period, when a test is more likely to be right. | | period | Expected period | The day your next period should start. | | ovulation | Ovulation | The ovulation day: entered, or 14 days before the expected period. | | implantFrom | Implantation from | The earliest usual implantation day: 6 days after ovulation. | | implantTo | Implantation up to | The latest usual implantation day: 12 days after ovulation. | ## Method Expected period = last period + cycle length (or ovulation + 14 days); test on the expected period, more reliably 7 days later; implantation 6 to 12 days after ovulation. From a day of sex: test 21 days later. ## Assumptions - Ovulation comes 14 days before the expected period (ACOG average); real cycles vary. - A test cannot be positive before implantation, which is usually 6 to 12 days after ovulation, most often 8 to 10 (Wilcox and others, 1999). - Even with a very sensitive test, about 10% of pregnancies are not yet detectable on the first day of the missed period; 1 week later, at most 3% are not (Wilcox and others, 2001). - Home tests differ in sensitivity. Follow the instructions on your test, and see a health care professional for a blood test or advice. This is not medical advice. ## Worked examples 1. from = period, lmp = 2026-09-01, cycle = 28 gives test = 2026-09-29, reliable = 2026-10-06, period = 2026-09-29, ovulation = 2026-09-15, implantFrom = 2026-09-21, implantTo = 2026-09-27. Source: hand calculation in content.mdx: ACOG (28-day cycle, ovulation 14 days before the period); NHS (test from the first day of a missed period); Wilcox 1999 (implantation 6 to 12 days after ovulation); Wilcox 2001 (1 week later). 2. from = ovulation, ovulation = 2026-10-10 gives test = 2026-10-24, reliable = 2026-10-31, implantFrom = 2026-10-16, implantTo = 2026-10-22. Source: hand calculation in content.mdx: expected period = ovulation + 14 days (ACOG); Wilcox 1999 and 2001. 3. from = sex, sex = 2026-09-20 gives test = 2026-10-11. Source: NHS, Doing a pregnancy test: at least 21 days after unprotected sex; September 20 + 21 days. 4. from = period, lmp = 2026-01-01, cycle = 35 gives test = 2026-02-05, ovulation = 2026-01-22. Source: hand calculation in content.mdx: a 35-day cycle, so ovulation on day 21 and the period on day 35. ## FAQ ### How soon can I take a pregnancy test? The NHS says most tests can be used from the first day of a missed period. If you do not know when your period is due, test at least 21 days after you last had unprotected sex. A test cannot turn positive before the embryo implants, which is usually 6 to 12 days after ovulation. ### Why can a test be negative on the day of my missed period? Because implantation does not always happen before then. Wilcox and others (2001) found that about 10% of pregnancies had not implanted by the first day of the missed period, so even a perfect test could miss them. One week later, at most 3% were still undetectable. ### What is implantation and when does it happen? Implantation is when the embryo attaches to the lining of the uterus and starts making hCG, the hormone pregnancy tests detect. In a study of pregnancies that lasted at least 6 weeks, it happened 6 to 12 days after ovulation, and in 84% of them on day 8, 9, or 10 (Wilcox and others, 1999). ### I got a negative result but my period has not come. What now? Test again in a few days to a week, when more pregnancies can be detected. Irregular cycles, a late ovulation, or testing too early can all give a negative result. If your period still does not come, talk with a health care professional. ### Is a blood test more accurate than a home test? A blood test at a clinic measures hCG directly, but it still cannot detect a pregnancy before implantation. Ask a health care professional if you need an early or exact answer. ### Does the time of day matter? Follow the instructions that come with your test, including any advice on the time of day to test. ## Sources - NHS, Doing a pregnancy test (page last reviewed February 9, 2022). https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/ - Wilcox AJ, Baird DD, Dunson D, et al. Natural limits of pregnancy testing in relation to the expected menstrual period. JAMA 2001;286:1759–1761. doi:10.1001/jama.286.14.1759 - Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. New England Journal of Medicine 1999;340:1796–1799. doi:10.1056/nejm199906103402304 - American College of Obstetricians and Gynecologists, Fertility Awareness-Based Methods of Family Planning (FAQ024, last reviewed February 2025). https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning