The three formulas
All conception-date math starts from one fact: pregnancy is dated at 280 days (40 weeks) from the first day of the last menstrual period, and conception happens about 266 days (38 weeks) before the due date. So you can work backward from whichever date you know.
From your due date: conception date = due date minus 266 days. This is the most useful direction if your provider gave you a due date at the first ultrasound, since that date is usually the most reliable anchor.
From your last period: conception date = first day of last period plus 14 days. This assumes ovulation around day 14 of a 28-day cycle. If your cycles run longer, shift the estimate by the difference: a 32-day cycle typically ovulates around day 18, so add 18 days instead of 14.
From an ultrasound: a first-trimester scan gives a gestational age, say 8 weeks 3 days. Conception happened 14 days after the pregnancy "started," so conception date = scan date minus (gestational age in days minus 14). A scan on March 20 showing 8 weeks 3 days (59 days) means the pregnancy started January 21 and conception was around February 4.
Worked example: from a due date
Suppose your due date is October 22, 2026. Subtract 266 days: October 22 minus 266 days lands on January 29, 2026. (Check it: January 29 to October 22 is 266 days.) That is the estimated conception date.
Now cross-check against the last period. Add 14 days back: January 29 minus 14 days is January 15, so the implied last period started January 15, 2026. And January 15 plus 280 days is October 22, which closes the loop. When both directions agree, the estimate is as solid as calendar math gets.
Worked example: from a last period
Suppose the first day of your last period was January 15, 2026, with a typical 28-day cycle. Add 14 days: conception around January 29, 2026. Then the due date is January 15 plus 280 days, October 22, 2026, and the conception date sits 266 days before that, which confirms the same January 29 answer.
If your cycle was 32 days instead, ovulation likely fell around day 18 rather than day 14, so conception would be around February 2, 2026, and the due date shifts four days later to October 26, 2026. That four-day shift is exactly the cycle-length difference.
Worked example: from an ultrasound
Suppose a scan on March 20, 2026 measures the pregnancy at 8 weeks 3 days. Convert to days: 8 times 7 plus 3 equals 59 days of gestational age. Subtract 14 days for the pre-ovulation portion: 59 minus 14 equals 45 days since conception. March 20 minus 45 days is February 3, 2026.
Early ultrasounds measure the embryo's crown-rump length, and in the first trimester that measurement dates a pregnancy within about a week. That is why providers often revise the due date after the first scan: the ultrasound number replaces the calendar guess.
Why it is a window, not a day
Calendar math gives you a single date, but biology gives you a range. Sperm can survive inside the reproductive tract for up to 5 days, and the egg survives roughly 24 hours after ovulation. So conception could have happened any time in the roughly 5 days leading up to ovulation.
For our January 29 example, the realistic window is roughly January 25 through January 30. Any single date the calculator shows you is the middle of that window. If you are trying to pin down a specific day for personal reasons, the biology simply does not cooperate: the best anyone can do is the window.
What a conception date cannot tell you
A conception date is an estimate built on averages: a 14-day follicular phase, a standard 280-day pregnancy. Real cycles vary, implantation takes 6 to 12 days after fertilization, and due dates themselves are estimates (only about 1 in 20 babies arrives on the exact due date). So treat the result as accurate within a few days, not to the hour.
One thing people often hope the date will settle is timing around two possible dates of intercourse close together. The 5-day window means dates a few days apart cannot be separated by math alone. That is a question for a provider, not a calculator. The pregnancy calculator will run all three of these formulas for you from whichever date you know.