Implantation Calculator
Implantation happens 6 to 12 days after ovulation, most often on day 8, 9 or 10. See those days for your cycle.
The tool
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Implantation timing by cycle length
Counted in cycle days, where day 1 is the first day of your last period. Implantation follows ovulation, so a longer cycle moves it later. In every row the window is the same 7 days after ovulation, and your period is due on day 1 of the next cycle.
| Cycle length | Ovulation | Implantation window | Most likely |
|---|---|---|---|
| 21 days | day 7 | days 13–19 | days 15–17 |
| 22 days | day 8 | days 14–20 | days 16–18 |
| 23 days | day 9 | days 15–21 | days 17–19 |
| 24 days | day 10 | days 16–22 | days 18–20 |
| 25 days | day 11 | days 17–23 | days 19–21 |
| 26 days | day 12 | days 18–24 | days 20–22 |
| 27 days | day 13 | days 19–25 | days 21–23 |
| 28 days | day 14 | days 20–26 | days 22–24 |
| 29 days | day 15 | days 21–27 | days 23–25 |
| 30 days | day 16 | days 22–28 | days 24–26 |
| 31 days | day 17 | days 23–29 | days 25–27 |
| 32 days | day 18 | days 24–30 | days 26–28 |
| 33 days | day 19 | days 25–31 | days 27–29 |
| 34 days | day 20 | days 26–32 | days 28–30 |
| 35 days | day 21 | days 27–33 | days 29–31 |
| 36 days | day 22 | days 28–34 | days 30–32 |
| 37 days | day 23 | days 29–35 | days 31–33 |
| 38 days | day 24 | days 30–36 | days 32–34 |
| 39 days | day 25 | days 31–37 | days 33–35 |
| 40 days | day 26 | days 32–38 | days 34–36 |
| 41 days | day 27 | days 33–39 | days 35–37 |
| 42 days | day 28 | days 34–40 | days 36–38 |
| 43 days | day 29 | days 35–41 | days 37–39 |
| 44 days | day 30 | days 36–42 | days 38–40 |
| 45 days | day 31 | days 37–43 | days 39–41 |
Window = 6 to 12 days after ovulation; most likely = 8 to 10 days after (84% of the pregnancies that continued in Wilcox 1999). Ovulation = cycle length minus 14.
How this 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 behind it
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).
In the ProbaBaby app
iPhone · iPad · Apple WatchPin down your ovulation day, and every date after it moves with it
Implantation and test days are only as good as the ovulation day under them. In the app's free Planning module, your own logs can confirm ovulation instead of leaving it to the calendar.
The app also
- Log ovulation tests and basal body temperature; a confirmed ovulation re-centres your fertile window.
- Four signals in one ovulation check (calendar, temperature shift, ovulation-test peak, egg-white mucus), with a confidence level.
- Short guides in the Learn tab, including implantation and when to test, with their sources named.
- The Planning module is free: no credits and no paywall.
PDF reports for the doctor visit, pediatrician visit, prediction, birth plan, birth decisions, cycle, fertility and the Period Tracker’s postnatal check, shareable by QR code (24 hours, one view). There is no CSV export from the baby log.
On the App Store
Frequently asked questions
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.
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.
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.
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.
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.
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.
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