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Conception Calculator

Estimate when your baby was conceived from your due date or the first day of your last period, with the likely fertile window and key pregnancy dates.

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This is an estimate based on a typical 40-week pregnancy and ovulation about 14 days before the next period. It cannot say exactly when conception happened or prove paternity. An early ultrasound gives the most accurate dating.

Quick Answer

Conception usually happens about 2 weeks after the first day of your last period, which is about 266 days, or 38 weeks, before your due date. A due date of December 6, 2026 points to conception around Sunday, March 15, 2026, with sex most likely to have led to pregnancy from Mar 10 to Mar 16.

Last updated: 2 November 2026 ·How we calculate this: due date and ovulation timing

Table of Contents

How This Conception Calculator Works

Choose whether you know your due date or the first day of your last period. From a due date, the calculator counts back 266 days, because pregnancy lasts about 38 weeks from conception. From your last period, it adds your cycle length minus 14 days to estimate ovulation. It then shows the likely conception window, since sperm can live for several days.

Quick explanation: doctors count pregnancy from your last period, about 2 weeks before conception.

  • Due date − 266 days: estimated conception.
  • Last period + 14 days: conception in a 28-day cycle.
  • Window: sex up to about 5 days before ovulation can lead to pregnancy.

How to Use the Conception Calculator

  1. Choose due date or first day of last period.
  2. Pick the date.
  3. If using your last period, enter your average cycle length.
  4. Press Calculate.
  5. Read the estimated conception date, the likely window and your due date.

Core Formulas

  • From due date: conception = due date − 266 days
  • From last period: conception = LMP + (cycle length − 14) days
  • Due date: LMP + 280 days + (cycle length − 28)
  • Window: 5 days before to 1 day after the estimated conception date

Key Terms

  • Conception: fertilization of an egg by a sperm.
  • Ovulation: the release of an egg, when conception becomes possible.
  • LMP: the first day of your last menstrual period.
  • Gestational age: pregnancy age counted from the LMP.
  • Fertile window: the days when sex can lead to pregnancy.

Worked Example

Your last period started on March 1, 2026, and your cycle is 32 days.

  • Conception ≈ March 1 + (32 − 14) days = Thursday, March 19, 2026
  • Due date ≈ March 1 + 280 + 4 days = Thursday, December 10, 2026
  • The likely window is Mar 14 to Mar 20

Real-Life Examples

  • Curious parents: a due date of December 6, 2026 means conception around Mar 15.
  • Long cycles: with a 35-day cycle, conception is about 21 days after the start of your last period, not 14.
  • Matching an ultrasound: if an early scan sets a new due date, enter it to update the conception estimate.
  • Planning a pregnancy: use our Ovulation Calculator to find upcoming fertile days.

Conception Date vs Due Date

  • Conception date: when the egg was fertilized, about 2 weeks after your last period.
  • Due date: 40 weeks after your last period, or about 38 weeks after conception.
  • Gestational age: counted from the last period, so it is about 2 weeks more than the baby’s age since conception.

Conception Dates by Due Date

Due dateEstimated conceptionLikely window
Nov 15Feb 22Feb 17 to Feb 23
Dec 6Mar 15Mar 10 to Mar 16
Jan 10Apr 19Apr 14 to Apr 20
Feb 14May 24May 19 to May 25
Mar 21Jun 28Jun 23 to Jun 29
Apr 25Aug 2Jul 28 to Aug 3

Why Exact Conception Dates Are Uncertain

  • Sperm survival: sperm can live in the body for several days, so sex days before ovulation can lead to pregnancy.
  • Ovulation varies: it does not always happen on the same cycle day.
  • Due dates are estimates: an ultrasound may adjust them.
  • Implantation: the embryo implants several days after conception.

Common Mistakes to Avoid

  • Treating the date as exact. It is an estimate within a window of several days.
  • Using it to prove paternity. Only a DNA test can do that.
  • Forgetting cycle length. Long or short cycles move ovulation.
  • Counting from the end of your period. Use the first day of bleeding.
  • Ignoring ultrasound dates. A dating scan is more accurate than period dates.

Frequently Asked Questions

Subtract 266 days from your due date, or add your cycle length minus 14 days to the first day of your last period.

It gives a good estimate, usually within a few days, but cannot pinpoint the exact day.

No. Conception windows overlap, so only a DNA test can confirm paternity.

It can be the same day, or up to several days later if sperm are waiting for ovulation.

About 2 weeks, because pregnancy is counted from the last period.

If the ultrasound changes your due date, the estimated conception date moves with it.

Estimates from your last period are less reliable. Use the due date from an early ultrasound if you have one.

The NHS says sperm can survive for up to 7 days after sex, though usually less. That is why the fertile window spans several days.

Several days after conception, once the fertilized egg has traveled to the womb. A pregnancy test turns positive only after implantation.

Most home tests work best from the day of your missed period.

Doctors date pregnancy from the last period, not conception, so the dates differ by about 2 weeks.

For IVF, use the transfer date and embryo age instead. Our Due Date Calculator supports IVF dates.

References & Sources

SourceUsed ForLink
ACOG Committee Opinion 700, Methods for Estimating the Due Date How due dates relate to the last period and conception. https://www.acog.org/clinical/guidance/committee-opinion/2017/05/methods-for-estimating-the-due-date
NHS, Periods and fertility in the menstrual cycle Ovulation timing and sperm survival. https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/