The due date is presented as a single day and functions as an anchor for the whole pregnancy. It is a midpoint of a distribution rather than a prediction of an event.
The traditional calculation makes assumptions
The conventional method counts forward from the first day of the last menstrual period, assuming a regular cycle of a standard length and ovulation at its midpoint.
Cycle length varies substantially between individuals and between cycles in the same person, and the timing of ovulation within a cycle varies as well.
Where either assumption does not hold, the calculated date can be out by a week or more, which is why the date is reviewed rather than fixed at the first appointment.
Early ultrasound is more reliable
Dating scans measure the embryo and compare the measurement to expected growth. Early in pregnancy, growth is consistent enough between pregnancies for this to be reasonably precise.
The accuracy of dating by measurement falls as pregnancy progresses, because individual variation in size increases. A scan later on says more about the baby's size than about the dates.
This is why services date the pregnancy early and generally do not revise it afterwards, even when later measurements differ from the expectation.
Birth timing is a distribution
Only a small proportion of babies arrive on the estimated date itself. Spontaneous labour occurs across a span of weeks around it, with first pregnancies tending slightly later.
The term period is defined as a range rather than a point for this reason, and outcomes differ across that range, which is why the exact week matters clinically.
Treating the date as a deadline produces avoidable distress in the final fortnight, when a pregnancy continuing past it is entirely within the expected pattern.
The date drives decisions
Gestational age determines the timing of screening tests, the interpretation of growth measurements, and thresholds for interventions at both ends of the range.
An inaccurate date therefore shifts everything that depends on it, which is the practical reason services invest effort in dating rather than accepting the calculated figure.
It also determines when a pregnancy is considered post-term and when discussions about induction typically begin, and the thresholds for that differ between countries and guidelines.
What the date does not tell you
The due date carries no information about how labour will start, how long it will take, or whether the baby will be large or small.
It is also not a target that can be influenced by activity or diet, despite the volume of suggestion to the contrary in late pregnancy.
Questions about dating, about a pregnancy continuing past the estimated date, or about induction are for the midwife or obstetric team, since the answers depend on the individual pregnancy.