Conception Date from Due Date

Estimated conception window = due date − 266 days, ± a few days for fertile window. Not medical advice.

Inputs

Result

Loading calculator…
—

How to use this calculator

  • Enter due date.
  • Read estimated conception date and window.

About this calculator

Working backwards from a due date: subtract 266 days for estimated conception. The window extends 5 days earlier (sperm survival) through 1 day later (egg survival). Useful when paternity questions arise — but this is a formula tool only, not clinical or legal evidence.

Frequently asked

How wide is the conception window?+
Usually 6 days: 5 days before through ~1 day after ovulation. Conception is most likely on the ovulation day or 1-2 days before.
Is this useful for paternity?+
Only as a starting estimate. DNA testing is the only reliable paternity determination.
Can the due date be off?+
Yes — ultrasound dating is more accurate than LMP. The 266-day backwards calculation has the same uncertainty as the original due date estimate.
Medical advice?+
No. This is a formula tool. Pregnancy timing decisions should involve a qualified provider.
What if I had IVF?+
IVF dating is precise (count back from embryo-transfer plus implantation offset). For natural conception, the calculator's window is the best non-medical estimate.

Related calculators

Due Date from Conception Calculator
Estimated due date = conception + 266 days. Not medical advice.
Gestation Age Calculator
Compute gestational age (weeks + days) from last menstrual period. Not medical advice.
Pregnancy Week & Trimester Calculator
Pregnancy week, trimester, and weeks-until-due from LMP. Not medical advice.
Ovulation Window Predictor
Predicted ovulation date and fertile window from LMP and cycle length. Not medical advice.
Menstrual Cycle Predictor
Predict the next 6 expected period start dates from a recent period and cycle length. Not medical advice.
Days Between Dates Calculator
Count the exact number of days between any two dates — all calendar days, weekdays only, or business days (skipping US federal holidays). Pick whether to include both endpoints or just the difference.

More tools you might like

Hand-picked tools that pair well with this one — same audience, same intent.

Due Date from Conception Calculator
Calc

Estimated due date = conception + 266 days. Not medical advice.

Gestation Age Calculator
Calc

Compute gestational age (weeks + days) from last menstrual period. Not medical advice.

Ovulation Window Predictor
Calc

Predicted ovulation date and fertile window from LMP and cycle length. Not medical advice.

Pregnancy Week & Trimester Calculator
Calc

Pregnancy week, trimester, and weeks-until-due from LMP. Not medical advice.

Menstrual Cycle Predictor
Calc

Predict the next 6 expected period start dates from a recent period and cycle length. Not medical advice.

Pregnancy Due Date Calculator
Health

Estimated due date from last menstrual period (Naegele's rule), first-trimester ultrasound (gestational age + remaining weeks), conception date, or IVF transfer. Shows trimester + ACOG term-week milestones.