Figures here are checked against the Jukic et al. ovulation-tracking study cited below.
The clearest evidence on how much a due date can be off comes from a study that could measure pregnancy length precisely, because it tracked confirmed ovulation dates instead of guessing from a last period. Jukic and colleagues followed 125 women trying to conceive naturally, timed ovulation directly, and found a median pregnancy length of 268 days from ovulation, close to but not identical to what Naegele's rule predicts, with a normal range spanning roughly five weeks even after excluding preterm births (Jukic et al., Human Reproduction, 2013).
What the 280-day estimate is actually measuring
A due date built on Naegele's rule assumes ovulation on day 14 of a 28-day cycle, described in full on the due date calculation guide. It is a useful anchor for scheduling prenatal care, not a prediction of a specific delivery day, and the research on natural variation backs that up directly.
How much a normal pregnancy actually varies
In the Jukic study, even pregnancies that reached full term and had no complications varied in length by close to five weeks from shortest to longest. That is a wider spread than most people expect from a single calculated date, and it is the reason a due date functions better as the center of a window than as a target.
First-trimester scans narrow the error, not the biology
ACOG Committee Opinion No. 700 puts first-trimester crown-rump length dating at an accuracy of about plus or minus 5 to 7 days for establishing gestational age (ACOG, 2017). That narrows the chance that the calculated date itself is wrong. It does nothing to narrow the separate, biological fact that pregnancies of identical, correctly measured gestational age still deliver on different days within a normal range, which is what the Jukic data describes.
Born early or late: what the pattern looks like
Population-level data generally shows first pregnancies trending a little longer than later ones, on average, but the spread within either group swamps that difference for any individual pregnancy. The practical takeaway is the same either way: a due date tells you roughly where the middle of the distribution sits, not which day you personally will land on.
What to do with a two-week window
Full term under ACOG's own categories spans 39 weeks 0 days through 40 weeks 6 days, described in the full-term guide, and most providers do not treat "past the due date" as urgent on its own until well into that window. If your due date and an ultrasound date disagree by more than the threshold in ACOG's redating table, that is worth asking your provider about directly; if they simply differ from what you expected, that is closer to the range this page describes.
Check where you land
See your estimated due date and current week, built from your own dates.