Conception date calculator: work back from a due date

Estimated conception window from a due date of Thu, Oct 8, 2026worked example

Sat, Jan 10, 2026 - Fri, Jan 16, 2026

Midpoint of the window: Thu, Jan 15, 2026the date the convention points at
Estimated due date: Thu, Oct 8, 2026
Days in the window: 7 days
Gestational age at the midpoint: 2w 0dgestational age counts from the last period, not from conception

The convention subtracts 266 days from the due date, because the 280-day due-date rule assumes ovulation on cycle day 14. Sperm can survive several days in the reproductive tract and ovulation timing varies, so what this arithmetic produces is a window a week wide - never a single identifiable day.

Realistic range

Sat, Jan 10, 2026 to Fri, Jan 16, 2026

Bounded by ACOG's own fertile window: five days before ovulation through one day after it. This calculator cannot narrow the window further, cannot establish paternity, and cannot rule a particular date in or out. Those questions need a DNA test, not a calendar.

Which date do you have?

Estimated due dateUse the date from your scan if you have one - it is the more accurate input.

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

What this cannot do. It cannot establish paternity, rule a particular date of intercourse in or out, or tell you which of two candidate days it was. The window is wider than the question, deliberately - and honestly, because that is the width the underlying convention actually supports. If a scan has dated the pregnancy, that date is the better one to work from.

The sources this defers to

ACOG Committee Opinion 700: Methods for Estimating the Due Date — the 280-day convention and its day-14 ovulation assumption, from which 266 follows
ACOG: Fertility Awareness-Based Methods of Family Planning — the fertile window that bounds the conception window shown above

Forward instead of backward: the due date calculator. What happened in the week after conception: the implantation window.

A conception date calculator subtracts 266 days from an estimated due date, or adds 14 days to the first day of a last period, following the convention ACOG publishes. It returns a conception window rather than a single day, because sperm survive for several days and ovulation timing varies between cycles.

Estimated conception date from the due dateconvention — conception date = EDD - 266 days. Verified 2026-07-30 against ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (The result is a window, not a day. Sperm can survive several days in the reproductive tract, so intercourse days earlier can produce a conception dated here, and ovulation timing itself varies.)
Estimated due date from the last menstrual period — EDD = first day of LMP + 280 days. Verified 2026-07-30 against ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (ACOG states this convention assumes a regular 28-day cycle with ovulation on day 14, and does not account for inaccurate recall of the last period, irregular cycle length, or variation in when ovulation actually happened. ACOG treats a first-trimester ultrasound as the most accurate way to establish gestational age, and reports that about half of women recall their last period accurately.)
Fertile window (ACOG patient guidance) — fertile window = ovulation - 5 days through ovulation + 1 day. Verified 2026-07-30 against ACOG, Fertility Awareness-Based Methods of Family Planning (patient FAQ) (The window is drawn around an estimated ovulation date, so it is only as good as that estimate. Widening it is the honest response to an uncertain ovulation date; narrowing it is not.)

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

Frequently asked questions

How do you work out the date of conception?

Subtract 266 days from the estimated due date. The 280-day due-date convention ACOG publishes assumes ovulation on day 14 of a 28-day cycle, and 280 minus 14 is 266. From a last period date instead, add 14 days, adjusted for your cycle length.

Can a conception date be exact?

No. The date is arithmetic from an assumed ovulation day, and sperm can survive several days in the reproductive tract, so intercourse days before the dated conception can be the one that resulted in it. This tool shows a window for exactly that reason.

Can this establish paternity or rule out a particular date?

No, and it should not be used to try. The window here is wide enough that it cannot distinguish between candidate dates, and no calculator can. Paternity is established by a DNA test, not by arithmetic.

Why does my ultrasound date differ from this one?

An ultrasound measures the pregnancy; this calculator measures your dates. ACOG treats a first-trimester ultrasound as the most accurate way to establish gestational age, and reports that about half of women recall their last period accurately. Where the two disagree, the scan is the better estimate.

Does conception happen on the day of ovulation?

Fertilisation happens within about a day of ovulation, which is why the two are dated together here. Implantation is later: Wilcox 1999 dated it 6 to 12 days after ovulation, with 84 percent on days 8, 9 or 10.

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The next number

No body metric means much on its own. Here is what this one feeds into.

Due date calculatorThe forward direction, with the term window and trimester dates.Implantation calculatorWhat happened in the week after the date above, according to the study that measured it.

Related calculators

Due date calculatorPregnancy dating Ovulation calculatorCycle and ovulation Implantation calculatorEarly pregnancy

Every formula on this page is listed with its source on the methodology page, and changes to any reference value are logged in the data changelog. Canonical URL: https://myhealthmywellness.com/conception-date-calculator