The due date, also known as the estimated date of confinement, is an estimation of when a pregnant woman will deliver her baby. While the due date is often estimated as a single date, only about 4% of births occur exactly on it — it is best treated as the center of a range.
The default method bases the calculation on the last menstrual period (LMP), assuming childbirth occurs on average 280 days (40 weeks) of gestational age from the LMP. Cycle length can also be factored in: a longer-than-average cycle shifts ovulation, and therefore the due date, later.
Due dates can also be estimated from an ultrasound (comparing fetal growth to typical growth curves), a known conception date, or an IVF embryo transfer date, which tends to be the most precise method since the exact transfer date is known.