# When was my conception date?

Estimates the day a pregnancy was conceived, and the days sex most likely happened, from the due date or from the first day of the last period and the cycle length.

- Page: https://www.acalculator.org/health/conception-calculator
- JSON spec: https://www.acalculator.org/health/conception-calculator.json
- Version: f457f5f7ac05

## Default answer

Example with the default inputs (What do you know? Due date, Your due date February 16, 2027) on the example date Tuesday, September 29, 2026: You most likely conceived around May 26, 2026, from sex between May 21, 2026 and May 26, 2026.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| from | What do you know? | Whether you know the due date or the first day of your last period. |
| due | Your due date | The estimated due date from your doctor or midwife. |
| lmp | First day of your last period | The first day of bleeding of the last period before the pregnancy. |
| cycle | Cycle length (days) | Days from the first day of one period to the first day of the next; 28 on average. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| conception | Likely conception date | The estimated day of ovulation and conception. |
| windowStart | Sex most likely from | Five days before the conception date; the likely window ends on the conception date. |
| dueDate | Due date | The due date: 266 days after conception, or the date you entered. |
| periodStart | Last period started | The first day of the last period: the date you entered, or 280 days before the due date. |

## Method

From a due date: conception = due date − 266 days. From a last period: conception = last period + (cycle length − 14) days, due date = conception + 266 days. Likely window = conception − 5 days to conception.

## Assumptions

- The due date is 280 days after the first day of the last period, which assumes a 28-day cycle with ovulation on day 14 (ACOG).
- Ovulation comes 14 days before the next period would have started, so a longer cycle moves conception later.
- Conception is taken as the ovulation day. Sex up to 5 days before can lead to it, because sperm can survive that long (Wilcox and others, 1995).
- An ultrasound-dated due date is more accurate than one from the last period; the result is only as good as the date you enter. This is not medical advice.

## Worked examples

1. from = due, due = 2027-03-01 gives conception = 2026-06-08, windowStart = 2026-06-03, periodStart = 2026-05-25, dueDate = 2027-03-01. Source: ACOG Committee Opinion 700 (due date = last period + 280 days, ovulation on day 14); hand calculation in content.mdx: March 1, 2027 − 266 days.
2. from = lmp, lmp = 2026-06-01, cycle = 28 gives conception = 2026-06-15, windowStart = 2026-06-10, dueDate = 2027-03-08. Source: ACOG Committee Opinion 700: a 28-day cycle ovulates on day 14 and the due date is 280 days after the last period; Wilcox and others 1995 (6-day window).
3. from = lmp, lmp = 2026-06-01, cycle = 35 gives conception = 2026-06-22, windowStart = 2026-06-17, dueDate = 2027-03-15. Source: hand calculation in content.mdx: ovulation 14 days before the next period (ACOG FAQ024), so day 35 − 14 = 21 after June 1.

## FAQ

### How do I work out when I conceived?

Count back 266 days (38 weeks) from your due date. A due date is set 280 days after the first day of the last period, and conception is taken as about 14 days after that period starts, so 280 − 14 = 266. A due date of March 1, 2027 points to conception around June 8, 2026.

### Can I find the exact day I got pregnant?

Not from dates alone. Conception happens around ovulation, but sex up to 5 days before ovulation can lead to pregnancy, because sperm survive that long. Wilcox and others (1995) found that nearly all pregnancies came from sex in the 6 days ending on ovulation day, so the calculator shows that window.

### What if my cycle is longer or shorter than 28 days?

Choose "Last period" and enter your cycle length. Ovulation usually comes about 14 days before the next period would start (ACOG), so in a 35-day cycle it falls around day 21 instead of day 14, and conception and the due date move 7 days later.

### Which is more accurate, the due date or my last period?

A due date confirmed by an early ultrasound is usually more accurate. ACOG notes that dating by the last period assumes a regular 28-day cycle with ovulation on day 14, and does not allow for recall errors or cycle differences.

### Why is my pregnancy counted from before I conceived?

By convention, pregnancy weeks are counted from the first day of the last period, about 2 weeks before conception. So at conception you are already said to be "2 weeks pregnant", and a pregnancy lasts 40 weeks from the last period, or 38 weeks from conception.

### Can this tell who the father is?

No. The dates are estimates with a window of several days, and real cycles vary. Only a paternity test can show who the father is.

## Sources

- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics and Gynecology 2017;129:e150–e154. doi:10.1097/aog.0000000000002046
- American College of Obstetricians and Gynecologists, Fertility Awareness-Based Methods of Family Planning (FAQ024, last reviewed February 2025). https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine 1995;333:1517–1521. doi:10.1056/nejm199512073332301