A calendar estimate is not a delivery prediction or a reason to alter care. Cycle variation, uncertain menstrual dates and assisted reproduction can change the dating method. ACOG redating criteria depend on gestational age and the size of the discrepancy; there is no universal one-week override rule.
Confirm the pregnancy’s assigned due date with the prenatal team.
Enter the first day of your last menstrual period, which starts the count of pregnancy at 40 weeks, or 280 days. Naegele's rule adds 280 days to that date, which assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, or you conceived after a known ovulation, your true due date may differ.
An early ultrasound gives a more accurate date and is often used to confirm or adjust it.
Due Date: formula
Estimated due date from first day of last period
| First day of last period | February 16, 2026 |
|---|---|
| Estimated due date | November 23, 2026 |
| Estimated conception date | March 2, 2026 |
Common error: Treating the estimated due date as a deadline rather than a midpoint of a normal range. Only a small share of births happen on the exact estimated date, and a full-term pregnancy spans several weeks on either side of it.
Due Date: changing First day of last period
| First day of last period | Estimated due date | Estimated conception date |
|---|---|---|
| February 2, 2026 | November 9, 2026 | February 16, 2026 |
| February 9, 2026 | November 16, 2026 | February 23, 2026 |
| February 16, 2026 (example) | November 23, 2026 | March 2, 2026 |
| February 23, 2026 | November 30, 2026 | March 9, 2026 |
| March 2, 2026 | December 7, 2026 | March 16, 2026 |
Due Date: individual input comparisons
| Input | Value | Estimated due date | Estimated conception date |
|---|---|---|---|
| First day of last period | February 2, 2026 | November 9, 2026 | February 16, 2026 |
| February 9, 2026 | November 16, 2026 | February 23, 2026 | |
| February 16, 2026 | November 23, 2026 | March 2, 2026 | |
| February 23, 2026 | November 30, 2026 | March 9, 2026 | |
| March 2, 2026 | December 7, 2026 | March 16, 2026 |
Due Date: values and definitions
| Quantity and definition | Value used |
|---|---|
| First day of last period | February 16, 2026 |
| Estimated due date | November 23, 2026 |
| Estimated conception date | March 2, 2026 |
Naegele's rule adds 280 days, 40 weeks, to the first day of the last menstrual period, a convention that assumes a textbook 28-day cycle with ovulation on day 14: it is a population-level estimate baked into obstetric practice precisely because it needs no additional information, not a measurement of any individual pregnancy's actual conception date.
Compare this estimate against an early ultrasound-based due date if one exists; ultrasound dating in the first trimester is generally considered more accurate than a last-menstrual-period calculation for anyone with an irregular cycle.
See how far along you are on the pregnancy week calculator, the conception date and IVF due date pages for other starting points, and the pregnancy weight gain page. The ovulation and period calculators cover cycle timing. Ask your provider to confirm the date at the first visit.
Understand the quantities in this calculation
Does adding 280 days determine the actual delivery day?
No. It produces a conventional estimated due date from the menstrual start. Birth timing varies.
Does the estimated conception day establish paternity?
No. A fixed calendar offset cannot determine a biological conception date or resolve paternity.
Should any ultrasound difference replace the date?
No. ACOG criteria depend on gestational age, dating evidence and discrepancy. The calculator cannot apply that clinical decision.
How is a due date calculated?
The day-count method adds 280 days, or forty weeks, to the first day of the last menstrual period. The calendar form of Naegele’s rule adds seven days and adjusts months; it can differ from an exact 280-day count because calendar months have different lengths.
How accurate is a due date?
An estimated due date is a planning date, not a forecast of an individual birth. Menstrual-date accuracy, cycle length and clinical dating affect the estimate. Review the established obstetric date with the clinician rather than treating a displayed calendar day as certain.
What if my cycles are not 28 days?
A last-period calculation based on a conventional 28-day cycle may not fit the actual timing of ovulation. Tell the clinician the cycle history and use the established clinical dating assessment; the calculator cannot resolve a discrepancy from dates alone.