How the Reverse Due Date Calculator Works
Two Reference Points, Two Numbers
Pregnancy is dated in two different ways, and a reverse calculator has to account for both. LMP dating counts from the first day of the last menstrual period and adds 280 days (40 weeks) — this is the standard clinical convention, per Naegele's rule. Conception dating counts from the actual day the egg was fertilised, which is roughly 14 days after LMP, and adds 266 days (38 weeks) to reach the same due date.
This calculator subtracts 266 days from your due date to estimate the conception date, and 280 days to estimate the LMP — the same two numbers used by obstetricians when reviewing records or referral notes.
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.
Target Conception Date by Desired Birth Month
Planning ahead rather than looking back? This reference table shows the approximate conception window needed for a baby to arrive in a given month, based on a standard 266-day gestation from conception. Actual delivery can fall 1–2 weeks either side of the due date, so treat these as guide ranges, not guarantees.
| 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 |
Common Situations for Reverse Dating
Reviewing an Old Record
Only an EDD is documented in a referral letter or discharge summary. Reverse-calculate the LMP and conception window to reconstruct the antenatal timeline.
Curious About Your Own Birthday
Enter your date of birth as the "due date" (adjust if you were born early or late) to see the approximate window you were conceived in.
Planning Around a Target Month
Use the birth-month table above to identify roughly when to aim for conception if you'd like your baby to arrive in a particular season or month.
Cross-Checking IVF/IUI Dates
For assisted conception, the actual transfer or insemination date is far more accurate than this calendar estimate — use it only as a sanity check.