A due date is an estimate of the day a pregnancy reaches 40 weeks from the last menstrual period, or 38 weeks from ovulation. It is a planning mark. It is not a prediction that labor will start that morning, and it is not a diagnosis.
A figure often repeated in prenatal references is that about 4% of babies are born on the exact due date, with many births in the two weeks on either side. Treat that as context for the window, not as a personal probability.
What the formula assumes
The pregnancy due date calculator adds 280 days to the first day of the last period when the cycle is 28 days. Each day the cycle differs from 28 moves the estimate by one day. That math assumes ovulation about 14 days before the next period.
If the cycle length varies, or the period date is uncertain, the 280-day result moves with the error. A known ovulation date plus 266 days, on the ovulation due date calculator, skips the day-14 assumption. Due date from ovulation or conception is that version with an example.
What usually replaces the calculator
An early ultrasound is the dating method clinicians use when it disagrees with the period math. This site does not interpret a scan. If a clinic has already assigned a due date, that date is the one to plan around.
Embryo-transfer pregnancies are dated from the transfer and the embryo age. The period and ovulation tools here do not cover that case.
How to read the number
Use the date as the middle of a range. How many days a pregnancy is shows why 280 and 266 describe the same model. How the due date is calculated has the worked period examples. The week calculator answers “how far along” from the same period date, which will move every day even when the due date stays put.