When was my conception date?
Enter your due date, or the first day of your last period, to see the likely day you conceived and the window of days when sex most likely led to the pregnancy.
- Likely conception date
- May 26, 2026
You most likely conceived around May 26, 2026, from sex between May 21, 2026 and May 26, 2026.
- Sex most likely from
- May 21, 2026
- Due date
- February 16, 2027
- Last period started
- May 12, 2026
Answer for the example date Tuesday, September 29, 2026. It changes to today's date when the page loads.
Likely conception date: May 26, 2026. You most likely conceived around May 26, 2026, from sex between May 21, 2026 and May 26, 2026.
When did it happen?
This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use
How to calculate
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.
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.
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.
- 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
Each example is checked against the calculator on every build.
- What do you know? Due date, Your due date 2027-03-01 gives Likely conception date 2026-06-08, Sex most likely from 2026-06-03, Last period started 2026-05-25, Due date 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
- What do you know? Last period, First day of your last period 2026-06-01, Cycle length (days) 28 gives Likely conception date 2026-06-15, Sex most likely from 2026-06-10, Due date 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)
- What do you know? Last period, First day of your last period 2026-06-01, Cycle length (days) 35 gives Likely conception date 2026-06-22, Sex most likely from 2026-06-17, Due date 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
How the conception date is worked out
All dates are calendar days.
From a due date. ACOG sets the due date 280 days after the first day of the last period, assuming ovulation on day 14 of a 28-day cycle. Conception is taken as the ovulation day, so:
- conception = due date − 266 days;
- last period started = due date − 280 days.
From the last period. Ovulation comes about 14 days before the next period would start (ACOG), so with a cycle of c days:
- conception = last period + (c − 14) days;
- due date = conception + 266 days.
With c = 28 this is the same as the due date rule above: last period + 14 + 266 = last period + 280.
Likely window for sex. Nearly all pregnancies come from sex in the 6 days that end on ovulation day (Wilcox and others, 1995), so the window runs from conception − 5 days to the conception date.
Assumptions
- Conception happens on the ovulation day. In practice, fertilization is within about a day of ovulation.
- Ovulation is 14 days before the next expected period, which is an average; real cycles vary.
- The result is only as accurate as the date you enter. A due date from an early ultrasound is better than one from the last period.
This page is for information only. It is not medical advice.
Worked examples by hand
Due date March 1, 2027. 266 days before it: March 1, 2027 back to March 1, 2026 is 365 days, so 266 days back is 99 days after March 1, 2026, which is June 8, 2026. The window starts 5 days earlier, on June 3. The last period started 280 days before the due date, on May 25, 2026.
Last period June 1, 2026, 28-day cycle. Conception: June 1 + (28 − 14) days = June 15, 2026. Window from June 10. Due date: June 15 + 266 days = March 8, 2027 (the same as June 1 + 280 days).
Last period June 1, 2026, 35-day cycle. Conception: June 1 + (35 − 14) = June 1 + 21 days = June 22, 2026. Window from June 17. Due date: June 22 + 266 days = March 15, 2027.
Other questions people ask
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.