# Pregnancy Conception Calculator

> Estimate the likely conception date and six-day fertile window from your due date or last period, based on 266 days from conception to due date.

Category: Health & fitness · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/pregnancy-conception-calculator

Conception usually happens around ovulation, about 266 days (38 weeks) before the due date. The fertile window is the six days ending on the day of ovulation.

Also called: Conception date calculator, When did I conceive calculator.

## Inputs
- I know my (options: Due date, Last period; default due)
- Due date (default 2027-01-15)
- First day of last period (default 2026-04-10)
- Usual cycle length (unit days; default 28; optional)

## Results
- Most likely conception date
- Fertile window (6 days ending at ovulation)
- Due date
- Pregnancy dated from (LMP equivalent)

## Formula

```
Ovulation/conception = due date − 266 days (= cycle-adjusted LMP + 14 days);  Fertile window = ovulation − 5 days to ovulation
```

Where:
- 266 days: 38 weeks from conception to the due date
- fertile window: the six days ending on ovulation (Wilcox et al., 1995)

## Worked example
1. Due date 15 January 2027 − 266 days = 24 April 2026 (estimated ovulation).
2. Fertile window: 5 days before, 19 April, to 24 April 2026.
3. Pregnancy is dated from 280 days before the due date: 10 April 2026.

**Result:** A due date of 15 January 2027 points to conception around 24 April 2026, from intercourse most likely between 19 and 24 April.

## What the result means

The date is an estimate with a margin of several days either way: ovulation timing varies from cycle to cycle, and sperm can survive for days before ovulation. A due date set by an early ultrasound gives a better estimate than one from period dates.

## Assumptions and limits
- Ovulation and conception 266 days before the due date (14 days after a cycle-adjusted LMP).
- Fertile window of six days ending on the day of ovulation (Wilcox, Weinberg and Baird, 1995).
- Cannot establish paternity or the exact day of intercourse.
- Less reliable with irregular cycles or a due date based only on period dates.

## Questions

### Can I find the exact day I conceived?

Not from dates alone. Intercourse up to five days before ovulation can lead to pregnancy, so several days are equally plausible.

### Is conception the same as implantation?

No. Implantation happens roughly a week after fertilisation; pregnancy tests turn positive after implantation.

## Sources
- [Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med, 1995](https://carta.anthropogeny.org/libraries/bibliography/timing-sexual-intercourse-relation-ovulation-effects-probability-conception)
- [ACOG Committee Opinion 700: Methods for Estimating the Due Date (2017)](https://www.healthcare.uiowa.edu/familymedicine/fpinfo/OB/OB2017/ACOG%20redating%20gestational%20age.pdf)

## Related calculators
- [Due Date Calculator](https://calculators.geekmidst.com/calculators/due-date-calculator)
- [Pregnancy Calculator](https://calculators.geekmidst.com/calculators/pregnancy-calculator)
- [Pregnancy Weight Gain Calculator](https://calculators.geekmidst.com/calculators/pregnancy-weight-gain-calculator)
- [Date Difference Calculator](https://calculators.geekmidst.com/calculators/date-difference-calculator)

_This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/pregnancy-conception-calculator