A due date feels like it should be a precise prediction, but it's built from a formula that makes a fairly broad assumption: that every pregnant person has a textbook 28-day menstrual cycle with ovulation occurring exactly on day 14. In reality, cycle length varies significantly between individuals and even cycle to cycle for the same person, which is exactly why due dates function as a statistical estimate rather than a guaranteed delivery date.
The formula: Naegele's rule
Naegele's rule, developed in the 19th century and still the standard approach today, calculates a due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). The logic behind the number: a full-term pregnancy is considered roughly 40 weeks from LMP, which accounts for about 2 weeks before ovulation (the assumed day-14 ovulation of a 28-day cycle) plus the standard 38 weeks of gestation counted from conception. The Pregnancy Due Date Calculator runs this exact calculation from your LMP date, and can also estimate a due date from a known conception date or an ultrasound measurement, which can adjust the estimate when actual cycle length differs from the 28-day assumption.
Why the actual assumption rarely holds exactly
Cycle lengths commonly range anywhere from about 21 to 35 days among healthy adults, and ovulation doesn't always occur precisely on day 14 even within a 28-day cycle. Someone with a 35-day cycle typically ovulates later, meaning a due date calculated purely from LMP using the day-14 assumption would run early relative to their actual conception date. This is part of why early ultrasounds, which measure fetal size directly, are often used to refine or confirm a due date, particularly when cycle length is known to be irregular.
What the statistics actually show about due dates
Research has consistently found that only a small percentage of babies, often cited as around 4-5%, are born on their exact calculated due date, with the majority arriving within a window of about a week or two before or after it. This isn't a flaw in the calculation so much as a reflection of the real biological variability in pregnancy length, which is why due dates are generally communicated (and should be understood) as an estimated window rather than a fixed appointment. Throughout pregnancy, tracking healthy weight gain alongside a due date estimate gives a fuller picture, and any specific concerns about timing or delivery should always go through a doctor or midwife who has access to your full medical history.