# Implantation Calculator _Implantation happens 6 to 12 days after ovulation, most often on day 8, 9 or 10. See those days for your cycle._ - Source: https://proba.baby/en/tools/implantation-calculator - Language: English - Last verified: v1.0.4 · 7 Sep 2026 - Last updated: 2026-10-11 ## How it works The short answer: implantation, when a fertilised egg attaches to the lining of the womb, happens 6 to 12 days after ovulation. In the study that measured it day by day, 84% of implantations in continuing pregnancies fell on day 8, 9 or 10 after ovulation (Wilcox 1999). On a 28-day cycle that is roughly cycle days 20 to 26, most likely days 22 to 24. Work it out from the first day of your last period and your cycle length, or switch to your ovulation day if you know it from an ovulation test or a temperature chart. You get the implantation window, the most likely days inside it and the earliest day a pregnancy test can work. Copy a link to come back to the same dates later. Implantation always shows as a range of days here, never as one day, because nobody can see it happen. There is no reliable sign you can feel on the day, and light spotting in early pregnancy has not been shown to come from implantation. A test can only turn positive after implantation, which is why the earliest test day is the day your period is due. These are calendar estimates: not a diagnosis and not contraception. ## The math From your last period: ovulation = first day of your last period + (cycle length − 14 − 1) days, the ProbaBaby app's rule. From a known ovulation day, that day is used as it is. Implantation window = ovulation + 6 days to ovulation + 12 days; most likely = ovulation + 8 to + 10 days. These are the range and the 84% core that Wilcox, Baird and Weinberg measured with daily urine hCG tests in women trying to conceive, in the pregnancies that lasted at least six weeks (N Engl J Med 1999). Earliest test day = the day your next period is due (the first day of your next cycle, or ovulation + 15 days from a known ovulation day); most reliable = 7 days later (Wilcox 2001). ## FAQ **When does implantation happen?** Between 6 and 12 days after ovulation, and most often 8 to 10 days after it (in 84% of the pregnancies that continued in Wilcox 1999). On a 28-day cycle with ovulation around day 14, that is about cycle days 20 to 26. **Can I feel implantation?** There is no reliable sign. Mild cramps, bloating and sore breasts are common in the days before a period whether or not you are pregnant, because the same hormone, progesterone, is high in both cases. A pregnancy test after your period is due tells you far more than any symptom. **Is spotting a sign of implantation?** Not on its own. Light bleeding is common in early pregnancy and usually harmless, but in a study that tracked it day by day, researchers found no support for the idea that implantation causes it (Harville 2003). If you could be pregnant, any bleeding is worth a call to your midwife or doctor (NHS); very heavy bleeding, or bleeding with severe pain, needs your emergency number. **How soon after implantation can I take a test?** Tests look for hCG, which only starts to rise after implantation and takes a few days to reach a level a home test can see. Testing on the day your period is due is the earliest most tests are meant for, and one week later is more reliable. **Why does this give a range and not one day?** Because implantation timing varies from pregnancy to pregnancy and cannot be seen or felt. A single date would claim a precision no one has. The range comes from a study that measured implantation day by day with urine tests. **When should I call a doctor?** 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, 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 - Harville EW, Wilcox AJ, Baird DD, Weinberg CR. Vaginal bleeding in very early pregnancy. Hum Reprod 2003;18:1944-7: https://europepmc.org/article/MED/12923154 - 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/ - NHS: Vaginal bleeding in pregnancy: https://www.nhs.uk/pregnancy/related-conditions/common-symptoms/vaginal-bleeding/ - ACOG: Early Pregnancy Loss: https://www.acog.org/womens-health/faqs/early-pregnancy-loss --- 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.