Is the estimated conception date exact?

No. It is the estimated ovulation date implied by the selected pregnancy dating method. Intercourse may have occurred several days earlier, and the result cannot establish paternity.

Which method should I use?

Use the due date established by your clinician when available. ACOG says first-trimester ultrasound is the most accurate way to establish or confirm gestational age.

What does the intercourse window mean?

It covers five days before the estimated conception date through that date, matching ASRM's six-day fertile-window definition. It is still an estimate and does not identify a specific act of intercourse.

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Pregnancy Conception Calculator

Estimate when conception likely occurred from your due date, last menstrual period, or an ultrasound — with the probable intercourse window, due date, and last-period date.

Maintained by CalculatorBuddy TeamUpdated September 29, 2026Editorial methodology

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Estimated conception date

January 15, 2026

Estimated result

Likely intercourse window: January 10, 2026 – January 15, 2026
Corresponding due date: October 8, 2026
First day of last period: January 1, 2026

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Estimated conception dateJanuary 15, 2026

Work backward from a clinician-provided due date, last menstrual period, or ultrasound measurement to estimate the conception date and the preceding six-day fertile interval.

Examples

Due date October 8, 2026

The estimated conception date is January 15, 2026, the fertile interval is January 10–15, and the equivalent 28-day-cycle LMP date is January 1.

Formula

From due date: conception = due date − 266 days
From LMP:      conception = LMP + cycle length − 14 days
From scan:     conception = scan date − gestational age + 14 days
fertile interval = conception − 5 days … conception

This is date arithmetic, not a paternity test or confirmation of fertilization. Use the clinician's documented estimated due date for medical care.

Sources

ACOG: Methods for Estimating the Due Date — ultrasound, LMP, and ART dating.
American Society for Reproductive Medicine: Optimizing natural fertility — six-day fertile-window definition.

Frequently asked questions

Is the estimated conception date exact?: No. It is the estimated ovulation date implied by the selected pregnancy dating method. Intercourse may have occurred several days earlier, and the result cannot establish paternity.
Which method should I use?: Use the due date established by your clinician when available. ACOG says first-trimester ultrasound is the most accurate way to establish or confirm gestational age.
What does the intercourse window mean?: It covers five days before the estimated conception date through that date, matching ASRM's six-day fertile-window definition. It is still an estimate and does not identify a specific act of intercourse.