Conception Date Calculator
Work backwards to the week it all began, as an estimate, because biology keeps its own calendar.
Curious when this all began? Enter your due date (or last period) and we’ll estimate your conception window, gently noting that biology doesn’t keep exact appointments.
How this works
Conception typically happens about 266 days (38 weeks) before the estimated due date, so we subtract 266 days from your due date and show a ±3-day window around it. If you enter a last period instead, we first estimate the due date with Naegele’s rule (ACOG Committee Opinion 700).
Sources & method
Due-date and gestational-age math follows ACOG Committee Opinion 700, “Methods for Estimating the Due Date” (Naegele’s rule and ART-based dating). Trimester boundaries follow ACOG patient-education definitions. All results are planning estimates, your prenatal healthcare provider determines your clinical due date.
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