Calculate from
How the conception date is calculated
Conception (fertilisation) happens at ovulation, which in a standard cycle occurs 14 days before the next expected period — so, for a cycle that isn't exactly 28 days, ovulation shifts earlier or later relative to the LMP by the same difference.
From LMP: Conception date = LMP + (Cycle length − 14) days
From Due Date: Conception date = Due Date − 266 days
Both routes are consistent with each other and with the standard 280-day (LMP-based) / 266-day (conception-based) pregnancy length.
Clinical use
- Estimating exposure timing: commonly used to estimate when a medication, illness, or environmental exposure occurred relative to conception, or for legal/paternity-adjacent timeline questions.
- Cross-checking dating: the estimated conception date should be consistent with a first-trimester ultrasound's dating — if they disagree substantially, ultrasound dating (see the OB Wheel) is more reliable than LMP-based estimates.
- Accuracy depends on cycle regularity: this estimate assumes a consistent luteal phase of 14 days, which is a reasonable population average but can vary by a few days for any individual cycle.
References
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129:e150–e154.
- Fehring RJ, Schneider M, Raviele K. Variability in the phases of the menstrual cycle. J Obstet Gynecol Neonatal Nurs. 2006;35:376–384.