The due date is the single number everyone asks for first, and the Naegele method produces it by adding 280 days to the first day of the last period, or 266 days to a known conception date. Add a reference date and the tool also reports how many weeks and days along the pregnancy is today, turning a future date into a present position on the timeline.
A target, not a deadline
Only about four babies in a hundred arrive on the estimated date itself, and the great majority land within a fortnight either side, which is why clinicians treat the figure as the centre of a window rather than a promise. Irregular cycles or an early scan may shift the centre, and that adjustment is normal.
Why the Naegele estimate and an ultrasound date can disagree
The Naegele method assumes a standard 28-day cycle and ovulation on day 14; a first-trimester ultrasound measures the embryo directly and adjusts for individual variation, which is why care providers often shift the due date after that scan even when the LMP-based estimate was entered correctly.
Naegele's rule and where the standard due-date formula comes from
The most common method for estimating a due date, sometimes called Naegele's rule, adds 280 days — 40 weeks — to the first day of the last menstrual period. It's named after the 19th-century obstetrician credited with popularizing the convention, and despite its age it remains the standard first-pass estimate used across most prenatal care today, later refined where needed by ultrasound measurement.
Why 'due date' doesn't mean a guaranteed delivery day
Only a small minority of babies are actually born on their calculated due date — full-term delivery is normally defined as a range spanning several weeks around that date, not a single expected day. The due date is best understood as the estimated midpoint of a normal delivery window rather than a firm prediction of the actual delivery day.
Related reading: the Gestational Age Calculator covers the adjacent case in more depth.
Why the estimate can shift after an early ultrasound
As covered on the gestational-age page, an early ultrasound measurement is often more precise than a last-menstrual-period-based estimate, particularly for those with irregular cycles, and providers will frequently adjust the calculated due date based on that ultrasound finding. If your care provider has given you an ultrasound-adjusted due date, that figure should be treated as the more current and reliable one.
Full term is itself a range, not one date
Modern obstetric guidance defines 'full term' as a window — generally 39 to 40 completed weeks — distinguished from 'early term,' 'late term,' and 'post-term' categories, each associated with slightly different clinical considerations. The single due-date figure this calculator produces sits at the center of that broader window rather than representing the only day considered full term.
Why some due-date calculators ask for cycle length
A due-date estimate based purely on the last menstrual period assumes a standard 28-day cycle with ovulation around day 14; entering your actual average cycle length, where it differs from 28 days, allows for a more personalized estimate that better reflects your actual likely ovulation timing.
What 'estimated date of delivery' actually communicates to a care team
When a care provider records an estimated date of delivery in a prenatal chart, it functions as a planning reference for scheduling screenings, monitoring fetal growth against gestational age, and setting expectations for full-term timing — not as a firm prediction that delivery will occur on that specific day.
If this raises a follow-up question, the Conception Date Calculator is built to answer it.
How this estimate is used alongside growth scans
Later in pregnancy, growth ultrasounds compare the fetus's measured size against expected size for the calculated gestational age, which is one of the main ongoing uses of an accurate due-date estimate beyond the initial excitement of learning it.
Why 'full-term' babies still show a range of birth weights and readiness
Even within the defined full-term window, babies born earlier in that range versus later can show some differences in average birth weight and lung maturity, which is part of why the specific full-term sub-category (early, full, late term) is sometimes noted by a care provider alongside the general due date.
How this estimate interacts with elective induction scheduling
When an induction is scheduled for medical or elective reasons, it's typically planned relative to the estimated due date and current gestational age, making an accurate underlying due-date calculation directly relevant to that scheduling conversation with a care provider.
Why hospitals sometimes ask for the due date at every prenatal visit
Confirming the estimated due date at each prenatal visit helps a care team track gestational age consistently across the pregnancy and quickly catch any data-entry inconsistency in the medical record before it affects later clinical decisions.
Worth pairing this figure with the Baby Age in Weeks Calculator for a fuller picture.
A final note on emotional expectations around the due date
Many expecting parents find it helps to mentally treat the due date as the center of a multi-week window rather than a fixed appointment, which can ease some of the anxious anticipation that builds if the actual day of delivery, as is common, doesn't land precisely on the calculated date.
One more consideration for subsequent pregnancies
Some research suggests a person's own prior delivery timing relative to their due date can be a mild predictor for a subsequent pregnancy, though this is a general tendency rather than a reliable individual prediction.
A closing thought on flexibility around the calculated day
Most experienced parents and care providers alike encourage a degree of flexibility around the exact calculated day, since the surrounding weeks matter more in practice than the single date itself.