# Conception calculator: when did I conceive?

> Estimated conception date and fertile window from a due date, the last period or an ultrasound, with the likely range for each dating method.

Interactive version: https://www.calcopenly.com/health/conception-calculator
Subject: Health and fitness calculators

Conception is dated to ovulation, which in a textbook 28-day cycle falls 14 days after the first day of the last period. Working back from a due date, conception is 266 days earlier. From the last period it is the next expected period minus a 14-day luteal phase. From an ultrasound it is the scan date minus the gestational age measured on the scan, plus 14 days. The fertile window shown runs from 5 days before that date to 1 day after: Wilcox and colleagues (1995) saw pregnancies only from intercourse in the 5 days before ovulation or on the day itself.

People use it to work out when they got pregnant. A due date of 1 August 2026 points to conception around 8 November 2025, with intercourse most likely between 3 and 9 November.

Each method carries its own error. Luteal phases vary: in 612,613 recorded cycles the middle 95 % lasted 7 to 17 days, so period-based dates can be 3 days early or 7 days late. ACOG puts first-trimester scan dating within 5 to 7 days.

## Inputs

- **Work it out from** (options: Due date, First day of last period, Ultrasound scan)
- **Due date**
- **The due date was set from**: Sets the uncertainty: a first-trimester scan dates a pregnancy more tightly than the last period. (options: Last period, Early ultrasound)
- **First day of last period**
- **Usual cycle length**
- **Luteal phase length**: Days from ovulation to the next period; 14 is the textbook value.
- **Date of the scan**
- **Gestational age on the scan, weeks**
- **and days**

## Results

- Estimated conception date — main result
- Likely conception range, from
- Likely conception range, to
- Uncertainty of this method
- Fertile window, from
- Fertile window, to
- Estimated due date
- Gestational age today

## Formula

$$
\text{conception} = \text{due} - 266\ \text{d} = \text{LMP} + \text{cycle} - \text{luteal} = \text{scan} - \text{GA} + 14\ \text{d}
$$

## Worked examples

### Due 1 Aug 2026, dated from the last period

- Work it out from: Due date
- Due date: 2026-08-01
- The due date was set from: Last period
- **Estimated conception date: 2025-11-08**
- **Likely conception range, from: 2025-11-05**
- **Likely conception range, to: 2025-11-15**
- **Fertile window, from: 2025-11-03**
- **Fertile window, to: 2025-11-09**
- **Gestational age today: 11 weeks 5 days**
- **Estimated due date: not applicable**
- Checked against: Python 3.8 datetime: 2026-08-01 − 266 d; luteal 7–17 d (Bull 2019) gives −3/+7 d; Wilcox 1995 window −5/+1 d; today 2026-01-15 − 2025-10-25 = 82 d

### Due 1 Aug 2026, dated by an early ultrasound

- Work it out from: Due date
- Due date: 2026-08-01
- The due date was set from: Early ultrasound
- **Estimated conception date: 2025-11-08**
- **Likely conception range, from: 2025-11-01**
- **Likely conception range, to: 2025-11-15**
- **Uncertainty of this method: ±7 days**
- Checked against: ACOG CO 700: first-trimester crown–rump length accuracy ±5–7 days; Python 3.8 datetime ±7 d

### Last period 1 Dec 2025, 28-day cycle

- Work it out from: First day of last period
- First day of last period: 2025-12-01
- Usual cycle length: 28 days
- **Estimated conception date: 2025-12-15**
- **Likely conception range, from: 2025-12-12**
- **Likely conception range, to: 2025-12-22**
- **Fertile window, from: 2025-12-10**
- **Fertile window, to: 2025-12-16**
- **Estimated due date: 2026-09-07**
- **Gestational age today: 6 weeks 3 days**
- Checked against: Python 3.8 datetime: LMP + 28 − 14 = 2025-12-15; LMP + 28 − 17 and LMP + 28 − 7 (Bull 2019); due LMP + 280 (ACOG CO 700)

### Last period 1 Dec 2025, 32-day cycle

- Work it out from: First day of last period
- First day of last period: 2025-12-01
- Usual cycle length: 32 days
- **Estimated conception date: 2025-12-19**
- **Likely conception range, from: 2025-12-16**
- **Likely conception range, to: 2025-12-26**
- **Estimated due date: 2026-09-11**
- Checked against: Python 3.8 datetime: LMP + 32 − 14 = 2025-12-19; due LMP + 280 + 4 d

### Scan on 10 Jan 2026 at 8 weeks 3 days

- Work it out from: Ultrasound scan
- Date of the scan: 2026-01-10
- Gestational age on the scan, weeks: 8 weeks
- and days: 3 days
- **Estimated conception date: 2025-11-26**
- **Likely conception range, from: 2025-11-21**
- **Likely conception range, to: 2025-12-01**
- **Uncertainty of this method: ±5 days**
- **Estimated due date: 2026-08-19**
- Checked against: Python 3.8 datetime: 2026-01-10 − 59 d + 14 d; ACOG CO 700 Table 1: more than 5 days at ≤ 8 6/7 weeks

### Boundary: scan at exactly 9 weeks 0 days

- Work it out from: Ultrasound scan
- Date of the scan: 2026-01-10
- Gestational age on the scan, weeks: 9 weeks
- and days: 0 days
- **Estimated conception date: 2025-11-22**
- **Likely conception range, from: 2025-11-15**
- **Likely conception range, to: 2025-11-29**
- **Uncertainty of this method: ±7 days**
- Checked against: ACOG CO 700 Table 1: 9 0/7 to 13 6/7 weeks, more than 7 days; Python 3.8 datetime: 2026-01-10 − 63 d + 14 d

## Questions

### How do I calculate when I conceived from my due date?

Subtract 266 days, or 38 weeks, from the due date. The due date is 280 days after the first day of the last period, and in a 28-day cycle ovulation, and so conception, happens about 14 days after that period starts. A due date of 1 August 2026 gives a conception date of 8 November 2025. If the due date came from the last period, the true date is most likely 3 days earlier to 7 days later.

### How accurate is an ultrasound for dating conception?

Most accurate early in pregnancy. ACOG Committee Opinion 700 gives the crown–rump length measured up to 13 weeks 6 days an accuracy of ±5–7 days, ±7–10 days from 14 to 21 weeks 6 days, ±10–14 days from 22 to 27 weeks 6 days and ±21–30 days from 28 weeks. So a scan at 8 weeks dates conception to within about 5 days, and a third-trimester scan only to within 3 weeks.

### Can you get pregnant from sex a week before ovulation?

Not in the best-known data. In Wilcox and colleagues' 1995 study of 221 women trying to conceive, no pregnancy followed intercourse outside the six days ending on the day of ovulation. Inside that window the chance of conception rose from 10 % five days before ovulation to 33 % on the day itself. The fertile window shown here adds one day after ovulation as a margin.

### Why is my conception date two weeks after my last period?

Because pregnancy is counted from the first day of the last period, about two weeks before ovulation in a 28-day cycle. At conception a pregnancy is already called 2 weeks 0 days along. In longer cycles ovulation comes later: with a 35-day cycle and a 14-day luteal phase it falls on day 22, three weeks after the period started.

### How accurate is the conception calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 7 worked examples whose answers come from independent sources; for example, “Due 1 Aug 2026, dated from the last period” is checked against Python 3.8 datetime: 2026-08-01 − 266 d; luteal 7–17 d (Bull 2019) gives −3/+7 d; Wilcox 1995 window −5/+1 d; today 2026-01-15 − 2025-10-25 = 82 d.

### Where does the method come from?

ACOG Committee Opinion No. 700 — Methods for estimating the due date (2017), Table 1; Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med 2019;2:83; Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21.

## Sources

- [ACOG Committee Opinion No. 700 — Methods for estimating the due date (2017), Table 1](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date)
- [Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med 2019;2:83](https://doi.org/10.1038/s41746-019-0152-7)
- [Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21](https://doi.org/10.1056/NEJM199512073332301)

_Note: medical information, not professional advice._