Implantation Calculator
Estimate the implantation window and the earliest reliable day to take a pregnancy test.
About the Implantation Calculator
Enter the first day of your last period and your usual cycle length. The calculator estimates ovulation, the 6–12-day window when implantation can happen, the most likely days within it, and the first day a home test is most reliable.
After fertilisation the embryo implants in the uterine lining 6–12 days after ovulation, most often on days 8–10 (Wilcox et al., NEJM 1999). hCG becomes detectable in blood a few days later; home urine tests are most reliable from the first day of a missed period.
The formula
Ovulation ≈ LMP + cycle − 14; implantation ≈ ovulation + 6 to 12 days
Worked example
Last period started 20 October 2026, 28-day cycle
- Ovulation ≈ 20 October + 14 days = 3 November
- Implantation window: 9 to 15 November, most likely 11 to 13 November
- Most reliable test date: 17 November, when the next period is due
Things to keep in mind
- Based on average timings; ovulation can come earlier or later in any cycle.
Questions people ask
Is spotting implantation bleeding?
Light spotting around this time can be implantation, but it isn't reliable as a sign.
Why can a test be negative early?
hCG may not yet be high enough; retest in a few days.
Do I need a sensitive early test?
Early tests can detect lower hCG, but a negative before your period is due is often just too early.