How conception is estimated
Due dates are set 266 days after conception, so working backwards, estimated conception is 266 days before the due date. That is the same as 2 weeks 0 days in the pregnancy count, 14 days after the first day of the last period in a 28-day cycle.1
Example: a due date of June 30, 2027 gives estimated conception on October 7, 2026. Intercourse that led to the pregnancy most likely happened between October 2, 2026 and October 7, 2026, if ovulation happened on the estimated day.
The fertile window
In a prospective study of couples trying to conceive, pregnancies came only from intercourse during the six days ending on the day of ovulation.2 That is why the result shows a six-day window rather than a single date.
The window is only as good as the ovulation estimate behind it. Even among people with regular cycles, the day of ovulation varies a lot from cycle to cycle and person to person.3 A due date from a first-trimester scan narrows things, but scan dating is still only accurate to within about 5 to 7 days.1
What this cannot tell you
Estimated conception timing cannot identify which sexual encounter caused a pregnancy or establish paternity. When encounters were days or even a couple of weeks apart, every source of uncertainty above can overlap them. If paternity matters, a DNA test is the only reliable answer.