Estimate a possible conception date from a due date or last menstrual period; a calendar tool cannot identify the exact day.
Estimated Conception Date
How the Conception Date Is Estimated
If you enter an estimated due date, the calculator subtracts 266 days (38 weeks). If you enter the first day of the last menstrual period, it adds 14 days as an approximate ovulation and conception date. It then shows an illustrative fertile window around that estimated date.
Why the date may be uncertain
Ovulation does not always happen on day 14, and sperm and egg survival, cycle variation, and clinical dating all affect estimates. A due date may also be revised after an ultrasound or clinician review.
Health note: A calendar calculation cannot identify the exact day of conception, establish paternity, or assess pregnancy health. Seek clinical guidance when dates matter for care.
Source and important context
ACOG describes pregnancy dating as an estimate that may be refined using ultrasound and clinical information. A calendar calculation cannot establish the exact day of conception. ACOG: Methods for Estimating the Due Date. This page provides general educational estimates and is not a diagnosis or a substitute for individualized care. Page information updated: 4 November 2026.