Conception Date Calculator
Estimate the day you most likely conceived from your due date or the first day of your last period.
Conception usually happens around ovulation, roughly two weeks after your last period started. The exact day is impossible to pin down, so treat the window as your best estimate.
How it works
Conception usually happens around ovulation, roughly two weeks after the first day of your last period in a typical 28-day cycle. That is why doctors count pregnancy from the last period rather than the day of conception itself — it is simply easier to pin down.
If you know your due date, this works backwards: subtract 266 days (38 weeks) to land near conception, or 280 days to reach the last period. Start from the last period instead and it adds about 14 days to estimate the fertile window.
These are estimates, not a stopwatch. Cycles vary, ovulation drifts, and sperm can survive several days, so treat the result as a likely window of a few days rather than a single guaranteed date.
Frequently asked questions
How accurate is a conception date?
It is an estimate with a few days of wiggle room. Ovulation timing shifts from cycle to cycle and sperm can live up to five days, so conception is really a short window rather than one exact day.
Why count from my last period?
The last period is a date most people can remember, while the exact moment of conception is invisible. Standard pregnancy dating uses the last period and assumes ovulation around day 14.
My cycle is not 28 days — does that matter?
Yes. Longer or shorter cycles move ovulation earlier or later, which shifts the conception estimate. If your cycle is regular but not 28 days, adjust your expectations by the difference.