When Did I Conceive? Working Back From a Due Date

The three ways to estimate it

You know	Calculation	Gives
Estimated due date	EDD − 266 days	Approximate conception date
Last menstrual period	LMP + 14 days	Approximate ovulation date
Birth date	Birth − 266 days	Rougher estimate

Worked through with a due date of 17 October 2026:

17 Oct 2026 − 266 days = 24 Jan 2026
matching an LMP of 10 Jan 2026

The two figures reconcile: 280 days from the last period, 266 from conception, with the 14-day gap being the wait for ovulation.

Estimate conception from a due date or birth date.Conception calculator →

Why it is a window, not a date

Two separate uncertainties stack up:

Ovulation timing. The day-14 assumption holds for a 28-day cycle. A 32-day cycle typically ovulates around day 18, moving every derived date by four days.
Sperm survival. Sperm remain viable for up to five days, so fertilisation can occur days after intercourse. The conception date and the date of intercourse are frequently different.

From last period to positive test

Day 1–14 follicular phase Day 14 ovulation, fertilisation Day 20–26 implantation Day 28+ test positive

Fertilisation, implantation and detection

Three separate events, days apart, which is why "when did it happen" has several possible answers:

Fertilisation occurs within about 24 hours of ovulation. This is the conception date a calculator estimates.
Implantation follows roughly 6 to 12 days later, when the embryo embeds in the uterine lining. Some people notice light spotting.
Detection becomes possible once hCG rises after implantation — typically around the time of a missed period, about four weeks after the last period began.

If you tracked ovulation, use that date. A recorded LH surge or a clear cervical mucus change is a far better anchor than a calendar formula, because it observed the actual event rather than assuming a textbook cycle.

What this cannot tell you

A calculated conception date carries a window of several days and cannot establish paternity. DNA testing is the only reliable method, and non-invasive prenatal tests using a maternal blood sample are available from roughly 7 to 9 weeks. Do not treat a calculator result as evidence either way.

When the dates disagree

If a calculated conception date does not match what you expected, the usual explanation is that ovulation did not happen on day 14 — long, short or irregular cycles all shift it. An early ultrasound measures the embryo directly and is accurate to about ±5–7 days, which is better than any calendar method. Where a scan and the dates disagree by more than that, clinicians go with the scan.

This is general information rather than medical advice; your clinician's dating is the one that counts.

Run your own numbers

Conception Due Date Ovulation

FAQ

How do I calculate my conception date?

Subtract 266 days from the estimated due date, or add about 14 days to the first day of your last period. Both approaches assume ovulation occurred on day 14 of a 28-day cycle, so treat the result as a centre point rather than a precise date.

How accurate is a conception date calculator?

It gives a reasonable estimate with a genuine margin of a few days either side. Ovulation timing varies between people and between cycles, and sperm surviving up to five days means intercourse and fertilisation can happen on different days.

Is the conception date the same as the day of intercourse?

Not necessarily. Fertilisation happens when sperm meets egg, which can be up to five days after intercourse. This is why the conception date and the date something happened are often not the same day.

Can a conception date determine paternity?

No. Calculated conception dates carry a window of several days, which is rarely narrow enough to distinguish between possibilities. DNA testing is the only reliable way to establish paternity, and non-invasive prenatal paternity tests are available from around 7 to 9 weeks.

Why does my conception date not match when I think I conceived?

Usually because ovulation did not fall on day 14. Longer cycles push ovulation later and shorter cycles earlier, which shifts every date derived from the standard assumption. An early ultrasound gives a better anchor than any calendar calculation.

Sources

Primary references used for the figures and rules on this page.

Methods for Estimating the Due Date — ACOG
Your Menstrual Cycle — Office on Women's Health