Conception Date Calculator

The estimate is calculated as birthdate minus 266 days (38 weeks). A full-term pregnancy averages 40 weeks measured from the last menstrual period (LMP), and conception typically occurs about 14 days after the LMP — hence 40 weeks minus 2 weeks = 38 weeks, or 266 days, before birth.

It is an educational estimate with a typical range of ±7 days (259–273 days before birth). Premature births mean conception happened later than the midpoint suggests; post-term births mean it happened earlier. Only cases with a known exact ovulation day or an IVF transfer date can pin down conception precisely.

Normal full-term gestation varies by about ±2 weeks, and the day of ovulation within a cycle varies from person to person. The tool therefore shows a likely range (birthdate − 273 to birthdate − 259 days) around the single most-likely midpoint date.

Very close: conception usually happens within about 24 hours of ovulation, since a released egg remains viable for roughly a day and sperm can survive several days. If you want to estimate ovulation and fertile windows from cycle data instead, use our ovulation calculator.

Scope boundary: this tool (conception-date) works backwards from a known birthdate — it is retrospective, answering "when was I conceived?" The separate conception-calculator tool works forwards from a due date or last menstrual period — it is prospective, answering pregnancy-planning questions like "when did conception happen / when is the due date?"

No. This is an educational estimate based on population averages and cannot determine an exact day of conception, paternity, or any medical fact. For personal medical questions, consult a qualified healthcare professional.

The alternate mode takes an estimated due date (which is itself an LMP-based estimate) and subtracts the same 266 days: conception ≈ due date − 266 days, with the same ±7-day range. To estimate a due date from an LMP date first, use our pregnancy due date calculator.