Pregnancy Conception Calculator
Estimated Conception Date--
Conception Window--
When did conception actually happen? This calculator answers that by working backward from either an established due date or the first day of the last menstrual period, returning the most likely conception day plus a few days of uncertainty on each side. Conception is taken to occur about 266 days before the due date, or roughly two weeks after the last period began, reflecting the gap between menstrual dating and the moment of fertilisation.
Formula
conception = dueDate − 266 (or lastPeriod + 14 + (cycleLength − 28))
- dueDate
- Estimated date of delivery, if known
- 266
- Days from conception to birth (38 weeks of embryonic age)
- lastPeriod + 14
- Approximate ovulation/conception day, adjusted for cycle length
How it works
- Choose your starting point: a known due date, or the first day of your last menstrual period together with your cycle length.
- From a due date, conception is set 266 days earlier (the 38-week embryonic age). From a last period, it is the start date plus 14 days, adjusted by how far your cycle differs from 28 days.
- A conception window of three days before to three days after the estimated date is shown, acknowledging that fertilisation can occur across the fertile span.
Worked example
A due date of December 1, 2026 is used to estimate conception.
- Subtract embryonic age: December 1 − 266 days = March 10, 2026.
- Window opens 3 days earlier: March 7, 2026.
- Window closes 3 days later: March 13, 2026.
Likely conception around March 10, 2026, within a window of March 7–13.
Frequently asked questions
- Why is conception 266 days before the due date, not 280?
- Due dates are dated from the last menstrual period (280 days), but actual conception happens roughly two weeks later at ovulation. Subtracting those 14 days leaves about 266 days of true embryonic development from conception to birth.
- Can this tell me exactly who the father is or the precise day?
- No. It gives a most-likely date and a several-day window, not a precise event. Because sperm can survive for days, the actual day of fertilisation can fall anywhere in that span, so it should not be treated as definitive.
- Which input is more accurate — due date or last period?
- If your due date was set by an early ultrasound, working back from it is usually the most reliable. The last-period method depends on regular cycles and a typical ovulation day, so it is more approximate.
- Does cycle length affect the conception estimate?
- When you start from the last period, yes — a longer cycle pushes the estimated conception day later, and a shorter one moves it earlier, because ovulation timing shifts with cycle length. Starting from a due date does not use cycle length.