How the conception date is estimated
Conception — the moment an egg is fertilised — usually happens within about a day of ovulation. In a typical 28-day cycle, ovulation happens around day 14, so the estimated conception date is 14 days after the first day of the last period, adjusted for your cycle length. If you start from a due date instead, the estimate is 266 days before the due date.
From your period: conception ≈ LMP + 14 days (+ cycle length − 28)
From your due date: conception ≈ due date − 266 days
Why intercourse and conception dates can differ
Sperm can survive in the reproductive tract for up to about five days, while an egg survives for roughly 12–24 hours after ovulation. That creates a six-day fertile window: the five days before ovulation and the day of ovulation itself. Intercourse on any of those days can lead to conception on the day of ovulation. This is why the calculator shows a fertile window as well as a single conception date — and why it is often impossible to pinpoint exactly which day a pregnancy began.
How precise is a conception date?
A conception date worked out from a period or due date is an estimate with a margin of several days, because ovulation timing varies from cycle to cycle. An early ultrasound narrows it down, and IVF gives an exact date. If you need to know conception timing for personal reasons, treat the result as the most likely date within a range, and talk to a healthcare professional for anything with medical or legal significance.
Related calculators
Already know your conception date? The Conception Due Date Calculator turns it into a due date. If you have a due date from your doctor, the Reverse Due Date Calculator shows every date that follows from it. And if you are curious about a birth that has already happened, the Birth Date Calculator estimates conception from a birthday.