How do you calculate conception date from a due date?

Subtract 266 days (38 weeks) from the estimated due date to get the estimated conception/ovulation date. Subtract 280 days (40 weeks) from the due date to get the estimated first day of the last menstrual period (LMP), since pregnancy is dated from LMP even though conception happens about two weeks later.

Is a reverse due date calculator accurate?

It gives a reasonable estimate, not an exact date. It assumes a textbook 28-day cycle with ovulation on day 14. Irregular cycles, delayed ovulation, and normal variation in pregnancy length (37–42 weeks) can shift the true conception date by several days to two weeks.

What is the difference between conception date and LMP?

LMP (last menstrual period) is the first day of the period before pregnancy and is the standard reference point for dating pregnancy. Conception occurs roughly 14 days after LMP, around ovulation. Due date is calculated as LMP + 280 days, which is the same as conception date + 266 days.

Can this calculator tell me the exact day I conceived?

No. It estimates a fertile window of about 6 days during which conception most likely occurred, based on average cycle timing. Only a documented ovulation trigger date, IUI/IVF transfer date, or early dating ultrasound can narrow this down more precisely.

Reverse Due Date Calculator: Find Conception Date

1How Reverse Due Date Dating Works

Obstetricians date pregnancy using two standard reference points. LMP dating counts 280 days (40 weeks) from the first day of the last menstrual period, following Naegele's rule. Conception dating counts 266 days (38 weeks) from the actual day of fertilization (which typically occurs around ovulation, ~14 days after LMP in a 28-day cycle).

To reverse-calculate: we subtract 266 days from the estimated due date (EDD) to find conception, and 280 days to find the likely LMP.

Why the Result Is a Window, Not a Single Day

Sperm can survive in the reproductive tract for up to 5 days, while the egg is viable for only 12–24 hours after ovulation. So intercourse any time in the 5 days before ovulation — or on the day itself — can result in conception.

That's why the calculator shows a 6-day fertile window ending on the estimated conception/ovulation day, rather than pretending to know the exact date. The true date depends on your actual cycle length, ovulation timing, and when fertilisation occurred within that window.

🔄If you know your ovulation trigger shot date, LH surge date, or IUI/IVF transfer date, that will always be more precise than a calendar-based reverse estimate.

2Target Conception Window by Desired Birth Month

Target Birth Month	Approximate Conception Window
January	April 10 – May 10
February	May 11 – June 7
March	June 8 – July 8
April	July 9 – August 7
May	August 8 – September 7
June	September 8 – October 7
July	October 8 – November 7
August	November 8 – December 8
September	December 9 – January 7
October	January 8 – February 7
November	February 8 – March 9
December	March 10 – April 9

3Common Clinical & Personal Scenarios

Reconstructing Medical Records

When only an EDD is available in a referral note, reverse-calculating the LMP and conception window helps reconstruct the gestational timeline for screening windows (e.g., NT scan, anomaly scan).

Assisted Reproduction (IVF / IUI)

For ART pregnancies, calendar estimates are secondary. The exact embryo transfer or IUI insemination date provides precise dating that supersedes Naegele's rule calculations.

4Frequently Asked Questions

Clinical Takeaway: Reverse calculations provide a 6-day fertile window preceding ovulation. Actual fertilization depends on sperm viability and exact ovulation timing within the cycle.

Medical Disclaimer: This reverse due date calculator is provided for informational and educational purposes only. Results are estimates based on average cycle statistics and Naegele's rule, and should not replace advice from a qualified healthcare provider.