How the estimate is worked out
ACOG counts pregnancy from the first day of the last period, 280 days to the average due date [1], and assumes it starts two weeks after that day [2]. So conception is about 266 days before the due date. From a last period, ovulation typically happens about 14 days before the next period starts, however long the cycle is [3], so conception is about the last period plus the cycle length minus 14 days.
From a due date: due − 266 days From a last period: LMP + (cycle − 14) days
The window runs from 5 days before that date to 1 day after it [4].
Example: from a due date
Due date Saturday, October 17, 2026: − 266 days = Saturday, January 24, 2026. Window: Mon, Jan 19, 2026 to Sun, Jan 25, 2026.
Example: from a last period
Last period Sat, Jan 10, 2026, 28-day cycle: + 14 days = Sat, Jan 24, 2026. With a 35-day cycle: + 21 days = Sat, Jan 31, 2026.
An estimate cannot establish paternity or show the exact day. If it matters, talk to your doctor or midwife.
Questions
Can this tell me the day conception happened?
No. It gives the most likely date and a window, nothing more. Sperm can live in the body for as long as 5 days and an egg survives about 12 to 24 hours after ovulation [3], so sex on several different days can lead to the same pregnancy.
Can it show who the father is?
No. A date range cannot show paternity. The window is several days wide, the estimate depends on a due date that is itself an estimate [6], and nothing here involves any test.
Why is it only approximate?
What is the window of days?
ACOG says you can become pregnant from sex anywhere from 5 days before ovulation until 1 day after [4]. The window runs from 5 days before the estimated date to 1 day after it.
Which is better to use, due date or last period?
Use the one you have more confidence in. If your care team gave you a due date after an ultrasound, use that, and ask them if you want to know more.