Conception calculator
Enter your due date, estimate when you conceived.
How do I find my conception date?
Count back 266 days from the due date, or forward 2 weeks from the first day of your last period. ACOG dating assumes ovulation on day 14 of a 28-day cycle, and pregnancy is assumed to start 2 weeks after the last period began.
Why is there a window, not one day?
Sperm can survive for several days, so sex from about 5 days before ovulation to 1 day after can lead to pregnancy — the fertile window used by ACOG.
How accurate is it?
It is an estimate. Cycles vary, and ovulation does not always happen on day 14, so the real date can differ by several days.
Can it tell who the father is?
No — the dates are too uncertain for that. Only a paternity test can.
What if I had IVF?
Then the transfer date is known exactly; use the due date calculator with the IVF option instead.
Worked example: from a last period to the conception window
Suppose the first day of the last period was March 1, 2026. Adding 280 days gives the due date, December 6, 2026. Counting back 266 days from the due date gives the estimated conception date, March 15, which is 14 days after the period began. The fertile window runs from 5 days before that date to 1 day after: March 10 to March 16. Every date is a straight count of days, so you can check any of them on a calendar.
Why can the real date differ by a week or more?
The calculator uses the ACOG convention that ovulation falls on day 14. The U.S. Office on Women’s Health notes that the part of the cycle before ovulation can vary from 13 to 20 days, and that a period follows ovulation within 14 to 16 days. A woman with a 35-day cycle therefore ovulates about a week later than the day-14 assumption, and her conception date would be later than the one shown. A shorter cycle moves it earlier. The calculator does not adjust for cycle length, so if you know your cycles are long or short, treat the result as a rough guide.
Is there another way to estimate the fertile days?
The Office on Women’s Health describes a calendar method based on your own cycles: subtract 18 from the length of your shortest cycle to find the first fertile day, and subtract 11 from the length of your longest cycle to find the last, counting the first day of your period as day 1. For cycles that range from 26 to 32 days that is day 8 to day 21. The same source says sperm can live up to three days in a woman’s body, while the ACOG window used above reaches back five days before ovulation; sources differ on how long sperm survive, which is one reason the window is a range rather than a single day.
Common mistakes with conception dates
Counting weeks of pregnancy from the conception date: clinicians count from the first day of the last period, so the baby is about two weeks younger than the stated gestational age. Using the day of intercourse as the day of conception, when sperm can wait for the egg. Counting from the end of the period instead of its first day. Assuming a 28-day cycle when yours is different. And using the result to settle who the father is: the dates are too uncertain for that.
Sources and related calculators
Sources: American College of Obstetricians and Gynecologists, Committee Opinion No. 700, “Methods for Estimating the Due Date” (the 280-day and day-14 convention used above); U.S. Office on Women’s Health, “Trying to conceive” (cycle phases, sperm survival, calendar method for fertile days). Related: due date calculator, ovulation calculator, pregnancy week calculator and period calculator.
For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.