By Zencalcy Editorial TeamLast reviewed About us
Conception is usually estimated rather than observed, because fertilisation happens within a narrow window that is rarely recorded. A due date can be converted back by subtracting about 266 days, while an LMP-based estimate adds roughly 14 days, adjusted for cycle length. This calculator supports both routes and shows the offset it used.
Estimate conception from a due date or last period.
From due date: conception ≈ due date − 266 days From LMP: conception ≈ LMP + 14 + (cycle − 28) days
The 266-day offset reflects the average interval from conception to birth, while 280 days is the conventional LMP-to-birth interval. The LMP method assumes ovulation about 14 days before the next period, adjusted for cycle length. Actual conception can occur a few days either side, and only ultrasound or fertility tracking can narrow it further.
| Symbol | Meaning |
|---|---|
| Due date | Estimated date of delivery. |
| LMP | First day of the last menstrual period. |
| Cycle | Typical cycle length used to adjust ovulation timing. |
Conception ≈ 25 December 2026 − 266 days = 3 April 2026. Using the LMP route, a last period on 1 January 2026 with a 28-day cycle gives conception ≈ 1 January + 14 days = 15 January 2026. A 35-day cycle moves the estimate to 22 January 2026.
Fertilisation can only happen in a narrow window around ovulation, but ovulation itself is not fixed. Sperm can survive for several days in the reproductive tract, and an egg remains viable for roughly a day. A single act of intercourse therefore does not map neatly onto a single conception date. Due dates add another layer of averaging: only a small share of babies arrive on the estimated date. When people need a more precise timeline, fertility treatment records, ovulation tracking or early ultrasound provide better anchors than calendar arithmetic. For general curiosity or paperwork, an estimate is usually enough. For medical decisions, rely on the dating your clinician has recorded.
Continue with the Pregnancy Weight Gain Calculator, Gestational Age Calculator, or the hCG Calculator — or browse more gynecology & pregnancy tools and all health calculators.
Also see the Ovulation Calculator and the Pregnancy Due Date Calculator.
It is a window rather than a precise moment. Even with a known due date, the interval from conception to birth varies between people and pregnancies. Use the result as an approximate range, not proof of a specific day.
Pregnancy dating and fertility estimates assume regular cycles and typical ovulation. They cannot confirm pregnancy, viability or gestational age, and they do not replace ultrasound or prenatal care. Seek urgent care for pain, bleeding or reduced fetal movement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
Content updated: September 2026