Conception Date Calculator
When a pregnancy most likely began, from a due date or the first day of the last period. You get the days it most likely happened, not a single date no calculation can know.
Work back from
From your clinician, a scan or the due date calculator.
Your dates stay on this device: nothing is sent, saved or added to the address bar.
ExampleA due date of Sat, Apr 24, 2027
Aug 1, 2026Saturday
The sex that led to it most likely happened between Jul 27 and Aug 1. Allow about a week either way: ovulation varies, and so does any due date.
Shaded: Jul 27 – Aug 1, the six days sex could have led to this pregnancy (Wilcox et al. 1995).
- Due date
- Sat, Apr 24, 2027
- Today
- 12 weeks 3 days
General information from published research, not a diagnosis or medical advice. Talk to a doctor before changing medication or treatment. A range from averages, not proof of when or with whom a pregnancy began. Terms
How this works: Method, 7 sources, Checked against 2 worked examples,
How this works
Method
A due date is set 280 days after the first day of the last period, and that convention assumes ovulation two weeks in, so conception is about 266 days before the due date. For a due date of Monday, June 21, 2027 that is Monday, September 28, 2026. Full method
Sources
- Wilcox, Weinberg and Baird, Timing of sexual intercourse in relation to ovulation (N Engl J Med, 1995)
- Jukic et al., Length of human pregnancy and contributors to its natural variation (Hum Reprod, 2013)
- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017, reaffirmed 2025)
- Committee Opinion No. 579: Definition of Term Pregnancy (ACOG, 2013)
- Predicting date of birth and examining the best time to date a pregnancy (Khambalia et al.) (Int J Gynaecol Obstet, 2013)
- Naegele’s rule: a reappraisal (Baskett and Nagele) (BJOG, 2000)
- Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (Bull et al.) (npj Digit Med, 2019)
How it’s tested
2 worked examples for this page are checked by automated tests before every release: given the inputs, the tool must show the expected answer.
Limits
General information from published research, not a diagnosis or medical advice. Talk to a doctor before changing medication or treatment. A range from averages, not proof of when or with whom a pregnancy began.
Changes
- First published: a due date from your last period, conception, IVF or a scan, with ovulation, conception, period and weeks-pregnant pages; nothing you enter leaves your device.
How the conception date is worked out
A due date is set 280 days after the first day of the last period, and that convention assumes ovulation two weeks in, so conception is about 266 days before the due date. For a due date of Monday, June 21, 2027 that is Monday, September 28, 2026.
Sex in the five days before ovulation can lead to pregnancy as well as sex on the day, because sperm survive that long, so the sex that led to it most likely happened between Wednesday, September 23, 2026 and Monday, September 28, 2026. That six-day span is the honest answer; the day of conception itself is rarely known.
If the due date came from an early scan, add its own uncertainty, about 5 to 7 days either way, to both ends. From a last period instead, the calculator first finds the due date (adjusting for cycle length) and works back the same way.
conception ≈ due date − 266 days; the sex that led to it ≈ that day and the 5 days before
- Pregnancies began only from sex in a six-day window ending on the day of ovulation; only 6% could be firmly traced to sperm three or more days old. Source: Wilcox, Weinberg and Baird, Timing of sexual intercourse in relation to ovulation (N Engl J Med, 1995).
- Measured from ovulation, the median pregnancy lasted 268 days (38 weeks 2 days), and healthy pregnancies still varied by 37 days. Source: Jukic et al., Length of human pregnancy and contributors to its natural variation (Hum Reprod, 2013).
- A first-trimester scan dates a pregnancy to within about 5 to 7 days either way; a third-trimester scan only to within 21 to 30 days. Source: ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017, reaffirmed 2025).
Likely conception dates for due dates in 2027
For a due date on the 15th of each month: the most likely conception date (266 days before) and the six days the sex that led to it most likely fell in.
| Due date | Conception, about | Most likely window |
|---|---|---|
| Fri, Jan 15, 2027 | Fri, Apr 24, 2026 | Apr 19–24, 2026 |
| Mon, Feb 15, 2027 | Mon, May 25, 2026 | May 20–25, 2026 |
| Mon, Mar 15, 2027 | Mon, Jun 22, 2026 | Jun 17–22, 2026 |
| Thu, Apr 15, 2027 | Thu, Jul 23, 2026 | Jul 18–23, 2026 |
| Sat, May 15, 2027 | Sat, Aug 22, 2026 | Aug 17–22, 2026 |
| Tue, Jun 15, 2027 | Tue, Sep 22, 2026 | Sep 17–22, 2026 |
| Thu, Jul 15, 2027 | Thu, Oct 22, 2026 | Oct 17–22, 2026 |
| Sun, Aug 15, 2027 | Sun, Nov 22, 2026 | Nov 17–22, 2026 |
| Wed, Sep 15, 2027 | Wed, Dec 23, 2026 | Dec 18–23, 2026 |
| Fri, Oct 15, 2027 | Fri, Jan 22, 2027 | Jan 17–22, 2027 |
Show all 12 rowsShow fewer
| Mon, Nov 15, 2027 | Mon, Feb 22, 2027 | Feb 17–22, 2027 |
| Wed, Dec 15, 2027 | Wed, Mar 24, 2027 | Mar 19–24, 2027 |
Frequently Asked Questions
Can this tell me who the father is?
No, and no date calculation can. The window is at least six days wide, ovulation varies from cycle to cycle, and dating adds days of uncertainty either way, so sex on dates a week or more apart can fit the same pregnancy. Only a DNA test can answer that question.
How accurate is a conception date from a due date?
Within about a week at best. The due date itself carries uncertainty (5 to 7 days either way from an early scan, more from a last period), and the 266-day count is an average. Read the result as “most likely in these days”.
Why is conception two weeks after my last period and not on it?
Pregnancy weeks are counted from the first day of the last period, but an egg can only be fertilised around ovulation, which in an average 28-day cycle comes about two weeks later. So at “4 weeks pregnant”, conception was about two weeks earlier.
I had IVF. Does this apply?
You likely know more than this page can work out: the transfer date and the embryo’s age date the pregnancy exactly. The due date calculator takes an IVF transfer directly.