# Conception Date Calculator

> Conception usually happens about two weeks after the first day of your last period, or 266 days before your due date. Enter your due date, last period or scan result to see the most likely day you conceived and the six-day window it fell in.

- Interactive calculator: https://www.leoella.com/tools/conception-date-calculator
- Plain-text version: https://www.leoella.com/tools/conception-date-calculator.md
- Category: Pregnancy
- Publisher: Leo&Ella (https://www.leoella.com)
- Last reviewed: October 19, 2026

## How it works

The calculator finds the first day of the pregnancy's dating (the last period of a 28-day cycle): 280 days before a due date, the period's start date moved by the cycle's difference from 28 days, or the scan date minus the weeks and days measured on an ultrasound. It puts conception 14 days after that date, which is 266 days before the due date, and shows the six days ending on it as the window in which the sex that led to the pregnancy most likely happened. It's an estimate: ovulation doesn't always fall exactly 14 days before a period.

## Key facts

- Conception is usually estimated at 266 days (38 weeks) before the due date: a due date is 280 days after the first day of the last period, and conception happens around 14 days after that period starts. [2]
- In Wilcox, Weinberg and Baird (NEJM, 1995), which tracked ovulation in 221 women with daily urine hormone tests, conception only followed sex in the six days ending on the day of ovulation. [1]
- In Wilcox et al. (NEJM, 1995), the chance of conceiving ranged from about 10% for sex five days before ovulation to about 33% for sex on the day of ovulation. [1]
- Cleveland Clinic describes the luteal phase, the time from ovulation to the next period, as about 14 days, with anywhere from 10 to 17 days considered normal. [3]
- An analysis of more than 600,000 app-tracked menstrual cycles (Bull et al., npj Digital Medicine, 2019) found an average luteal phase of 12.4 days, so ovulation 14 days before a period is a convention, not a rule. [4]
- ACOG says an ultrasound in the first trimester, up to 13 weeks 6 days, is the most accurate way to date a pregnancy. [2]

## Reference data

### From a due date

Conception is estimated 266 days before the due date; the window is the six days ending on that day.

| Due date | Estimated conception | Six-day window |
| --- | --- | --- |
| January 15, 2027 | April 24, 2026 | April 19, 2026 to April 24, 2026 |
| March 1, 2027 | June 8, 2026 | June 3, 2026 to June 8, 2026 |
| June 30, 2027 | October 7, 2026 | October 2, 2026 to October 7, 2026 |

### From a last period starting January 1, 2026

A cycle longer or shorter than 28 days moves the estimate by the difference (a common estimate, not a clinical rule).

| Cycle length | Estimated conception | Six-day window |
| --- | --- | --- |
| 24 days | January 11, 2026 | January 6, 2026 to January 11, 2026 |
| 28 days | January 15, 2026 | January 10, 2026 to January 15, 2026 |
| 32 days | January 19, 2026 | January 14, 2026 to January 19, 2026 |
| 35 days | January 22, 2026 | January 17, 2026 to January 22, 2026 |

## Frequently asked questions

### How do I calculate my conception date from my due date?

Count back 266 days (38 weeks) from your due date. A due date is 280 days after the first day of your last period, and conception usually happens around 14 days after that period starts, so conception is roughly 280 minus 14 days before the due date.

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

Usually not. Conception happens around ovulation, and ovulation can come earlier or later than day 14. Sperm can also survive for several days, so sex up to five days before ovulation can lead to a pregnancy. The calculator gives the most likely date and the six-day window that ends on it.

### Which is more accurate: dating from my period or an ultrasound?

An ultrasound in the first trimester (up to 13 weeks 6 days) is the most accurate way to date a pregnancy, according to ACOG. If you've had one, enter the due date or measurement from the scan.

### Can a conception calculator tell who the father is?

No. If there were two partners within a week or two of each other, dating can't reliably tell them apart, because the fertile window spans several days and ovulation timing varies. Only a DNA paternity test can establish paternity.

### How many weeks pregnant am I at conception?

About 2 weeks. Pregnancy is dated from the first day of your last period, roughly two weeks before ovulation, so you're counted as 2 weeks pregnant on the day you conceive.

### What is the fertile window?

The six days ending on the day of ovulation. In a study that tracked ovulation with daily hormone tests (Wilcox and colleagues, 1995), pregnancies only began from sex during those six days, with the chance ranging from about 10% five days before ovulation to about 33% on the day of ovulation.

## Sources

1. [Wilcox, Weinberg & Baird: Timing of sexual intercourse in relation to ovulation (NEJM, 1995)](https://www.nejm.org/doi/full/10.1056/NEJM199512073332301)
2. [ACOG Committee Opinion 700: Methods for Estimating the Due Date](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date)
3. [Cleveland Clinic: Luteal Phase](https://my.clevelandclinic.org/health/articles/24417-luteal-phase)
4. [Bull et al.: Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (npj Digital Medicine, 2019)](https://www.nature.com/articles/s41746-019-0152-7)

## About this page

This is general information for parents, not medical advice; a doctor, midwife or pediatrician who knows the family should guide individual decisions. The calculator runs in the visitor's browser and stores nothing they enter. When quoting this page, cite "Conception Date Calculator, Leo&Ella" and link https://www.leoella.com/tools/conception-date-calculator.