Naegele's rule: quantity example
Adding exactly 280 days to January 10, 2026 gives October 17, 2026. Adding 280 days to March 3, 2026 gives December 8, 2026; the shortcut of adding seven days and nine calendar months instead gives December 10. These operations are not identical for every starting date.
A common misunderstanding
Treating the calendar shortcut as exact 280-day arithmetic, or assuming any disagreement with a later ultrasound automatically changes an established clinical date.
Related measurements
LMP dating uses a menstrual reference; obstetric dating also considers reliable early ultrasound and assisted-reproduction dates. A clinician applies redating criteria rather than replacing the date after every scan.
Limits of the definition
The 280-day convention assumes a typical cycle relationship and cannot predict an individual birth date. Irregular cycles and uncertain LMP need clinical interpretation.
Understanding Naegele's rule
Naegele's rule turns one known date into an estimate by assuming a textbook cycle, which is precisely why it works the same for every pregnancy regardless of how the real cycle behaves: it is a population-level convention obstetric practice standardized on, not a measurement of any specific conception.
Using Naegele's rule
Providers and patients use it for a first estimate of the due date and to plan prenatal care. Calculators add the days for you and can show weeks and trimesters. Employers and families use the date to plan leave.
Dating helps time screening tests. As a first pass it is practical, and providers refine it with ultrasound and other findings.
Naegele's rule: follow the example through
Assumed example: The rule estimates a due date from the first day of the last menstrual period. Add 7 days, subtract 3 months, and add one year, while a direct 280-day count can differ from the calendar shortcut. If the last period began January 10, add 7 days to reach January 17, subtract three months to reach October 17 and the estimated due date falls on October 17 of the same year.
For January 10, 2026, the direct count also reaches October 17. For March 3, 2026, the calendar shortcut reaches December 10 while adding exactly 280 days reaches December 8, and it assumes a regular 28-day cycle with ovulation around day 14; the estimated date does not establish an individual birth date.
Naegele's rule: distinguish the related terms
Naegele's rule is a convention, not a measurement. Ultrasound dating can give a different estimate, especially in the first trimester, and clinicians often prefer it when it disagrees with the last-period date by more than a set margin; it also differs from conception dating, which counts from ovulation, conventionally about two weeks later in a 28-day cycle. Gestational age is counted from the last period, so a pregnancy at 8 weeks means about 6 weeks after conception; the due date is the point at 40 weeks, without identifying the date an individual birth will occur.
Naegele's rule: calculation reference
Naegele's rule: due date = first day of last period + 280 days; estimated conception ≈ +14 days
| Quantity | Entered or calculated amount | Meaning |
|---|---|---|
| First day of last period | February 16, 2026 | Explicit example input. |
| Estimated due date | November 23, 2026 | Result under the listed assumptions. |
| Estimated conception date | March 2, 2026 | Result under the listed assumptions. |
Due Date Calculator: change the inputs and compare the resulting cases
Is my due date exact?
No. It is an estimate. Only a small share of births happen on the due date, and most occur within a couple of weeks either side.
Your provider may adjust it after an ultrasound.
Weigh this estimate against an early ultrasound-based due date where one exists, since ultrasound dating is generally considered more reliable for an irregular cycle.