Pregnancy Conception Calculator
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Results
Estimated conception date Oct 15, 2026; the sex that led to the pregnancy most likely happened between Oct 10–15, 2026.
A due date set from an early scan is the most reliable starting point. If yours came from a period date, ovulation timing adds a few days of uncertainty either way.
A conception date can only be estimated: it cannot prove when, or with whom, a pregnancy started. If that matters, ask your doctor or midwife; paternity can only be confirmed by a DNA test.
Pregnancy dates are estimates, not medical advice. Follow the dates and care plan from your doctor or midwife, and talk to them about anything that worries you.
How to use the pregnancy conception calculator
- Choose what you know. A due date from an early dating scan is the most reliable; a period date or the scan details also work.
- Enter the date, plus your cycle length (for a period date) or the gestational age written on the scan report.
- Read the estimated conception date and the likely window. For a scan, the wider range allows for the scan's dating accuracy.
Conception date formula
conception ≈ first day of last period + (cycle length − 14) days
conception ≈ scan date − gestational age at scan + 14 days
likely window = conception − 5 days to conception
Due dates are set 280 days after the start of the last period, and conception is assumed two weeks into that count, so conception is 266 days (38 weeks) before the due date. Ovulation is assumed 14 days before the next period would have started. An egg can only be fertilized for about a day after ovulation, but sperm can survive for several days, so the sex that led to the pregnancy could have happened up to five days before ovulation. A landmark 1995 study by Allen Wilcox and colleagues found that pregnancies came only from sex in the six days ending on the day of ovulation.
Worked example
Your due date is Thursday, July 8, 2027. Counting back 266 days gives an estimated conception date of Thursday, October 15, 2026, and the likely window is Oct 10–15, 2026. If you only know that your last period started on October 1, 2026 and your cycles are 32 days, the estimate is Oct 19, 2026. A scan on November 26, 2026 that measured 8 weeks 0 days points to Oct 15, 2026, give or take about 5 to 7 days.
Conception dates for common due dates
| Due date | Estimated conception | Likely window |
|---|---|---|
| Jan 1, 2027 | Apr 10, 2026 | Apr 5–10, 2026 |
| Feb 1, 2027 | May 11, 2026 | May 6–11, 2026 |
| Mar 1, 2027 | Jun 8, 2026 | Jun 3–8, 2026 |
| Apr 1, 2027 | Jul 9, 2026 | Jul 4–9, 2026 |
| May 1, 2027 | Aug 8, 2026 | Aug 3–8, 2026 |
| Jun 1, 2027 | Sep 8, 2026 | Sep 3–8, 2026 |
| Jul 1, 2027 | Oct 8, 2026 | Oct 3–8, 2026 |
| Aug 1, 2027 | Nov 8, 2026 | Nov 3–8, 2026 |
| Sep 1, 2027 | Dec 9, 2026 | Dec 4–9, 2026 |
| Oct 1, 2027 | Jan 8, 2027 | Jan 3–8, 2027 |
| Nov 1, 2027 | Feb 8, 2027 | Feb 3–8, 2027 |
| Dec 1, 2027 | Mar 10, 2027 | Mar 5–10, 2027 |
How accurate is an ultrasound date?
ACOG gives these accuracy ranges for ultrasound dating; the earlier the scan, the narrower the range.
| Scan at | Dating accuracy |
|---|---|
| Up to 13 weeks 6 days | ±5 to 7 days |
| 14 weeks 0 days to 21 weeks 6 days | ±7 to 10 days |
| 22 weeks 0 days to 27 weeks 6 days | ±10 to 14 days |
| 28 weeks 0 days and later | ±21 to 30 days |
What the result can and can't tell you
- It is an estimate, not a fact. Ovulation varies from cycle to cycle, so even with a reliable due date the true conception date can be a few days either side.
- It can't settle paternity. When two possible dates are close, no calendar method can separate them. Only a DNA test confirms paternity.
- "Weeks pregnant" is not "weeks since conception". At 6 weeks pregnant, conception was about 4 weeks ago. The pregnancy calculator shows how far along you are.
This is not medical advice; talk to your doctor or midwife, who can review your scan dates with you.
Sources
- ACOG, AIUM and SMFM, Committee Opinion 700: Methods for Estimating the Due Date (2017).
- Wilcox AJ, Weinberg CR, Baird DD, "Timing of sexual intercourse in relation to ovulation", N Engl J Med 1995;333(23):1517–1521.
- NHS, Periods (fertility and ovulation timing).
Frequently asked questions
How do I work out when I conceived from my due date?
Subtract 266 days (38 weeks) from the due date. A due date of July 8, 2027 points to conception around October 15, 2026, with the sex that led to the pregnancy most likely between October 10 and 15.
Can I tell my conception date from my last period?
Roughly. Conception is assumed about 14 days before your next period would have started, so with a 28-day cycle it is 14 days after the period began, and with a 32-day cycle about 18 days after. Ovulation varies, so allow a few days either way.
Why is the conception window six days long?
Sperm can survive for several days, while an egg lasts about a day after ovulation. In a 1995 study, Wilcox and colleagues found pregnancies only came from sex in the six days ending on the day of ovulation, so that is the likely window.
How accurate is a conception date from an ultrasound?
ACOG puts first-trimester ultrasound dating at about ±5 to 7 days, falling to ±7 to 10 days from 14 to 21 weeks and wider later. The calculator widens the window by the scan’s accuracy.
Can a conception date prove paternity?
No. The date is an estimate, and ovulation timing varies, so two dates a few days apart cannot be told apart by calendar maths. Only a DNA test can confirm paternity.