A due date gets treated like an appointment -- something a baby is either on time or late for. But the number your calculator spits out was never built to predict an exact day. It's the midpoint of a normal range that spans weeks, and understanding the math behind it explains why so few births land on it exactly.
The standard formula: Naegele's Rule
Most due date calculators, including the one on this site, use a method called Naegele's Rule: take the first day of your last menstrual period (LMP) and add 280 days, or 40 weeks. That's it -- no ultrasound, no blood test, just a date and a fixed offset.
The 280-day figure comes from averaging two assumptions: a 28-day menstrual cycle, and ovulation occurring on day 14 of that cycle. Add roughly 266 days of gestation from conception to that, and you land on 280 days from the LMP. It's a clean, simple formula, which is exactly why it's been used for nearly two centuries -- and exactly why it's only an approximation.
Why the math breaks down for real cycles
The formula assumes everyone ovulates on day 14. In practice, cycle length varies a lot between people, and even from month to month for the same person. Someone with a 32-day cycle typically ovulates later than day 14, which means conception happens later than the formula assumes -- but the LMP-based calculation doesn't know that. It just adds 280 days regardless.
That mismatch is the main reason LMP-based due dates can be off by several days to a couple of weeks. It's not that the math is wrong; it's that the math is built on an average cycle that doesn't describe every person equally well.
Why ultrasound dating often overrides it
This is why a first-trimester ultrasound frequently resets the due date your LMP gave you. Early in pregnancy, fetal measurements -- specifically crown-rump length -- are remarkably consistent across different pregnancies at the same gestational age. There's much less individual variation in how fast an embryo grows in the first 12 weeks than there is in when any given person ovulates.
So if an early ultrasound measurement disagrees with the LMP-based due date by more than about a week, most providers will adjust the due date to match the ultrasound rather than the calendar math. It's a more direct measurement of fetal age than a memory of a period start date.
The due date is the middle of a window, not an edge
Obstetric guidelines don't treat 40 weeks as a deadline -- they define a whole range as normal. Roughly speaking: 37 to 38 weeks and 6 days is considered early term, 39 to 40 weeks and 6 days is full term, 41 weeks is late term, and 42 weeks and beyond is post-term. Your due date falls inside that full-term window, not at the end of it.
That's also why the commonly cited figure -- that only around 5% of babies are born on their actual due date -- isn't a sign that anything is wrong. The due date was always just the single most statistically likely day within a two-to-three-week spread on either side. Most births cluster in that spread, not on one specific date.
What the number is actually useful for
None of this makes the due date pointless. It's genuinely useful for scheduling prenatal visits on a consistent timeline, planning parental leave, figuring out when to pack a hospital bag, and giving your care provider a baseline to track growth and timing against. The mistake is treating it as a prediction instead of a planning anchor.
If you're trying to work out your own estimate, or figure out roughly where you are in a pregnancy based on the last period date, the Pregnancy Due Date Calculator runs the same LMP-plus-280-days math doctors use as a starting point. If you're earlier in the process and tracking cycles to figure out a fertile window, the Ovulation Calculator estimates that from your cycle length instead.