Pregnancy Due Date Calculator
Estimate a due date from the first day of the last menstrual period by adding 280 days (40 weeks).
About this calculator
Naegele’s rule dates a pregnancy at 280 days (40 weeks) from the first day of the last menstrual period. It is a planning estimate, not a diagnosis, and a clinician may adjust it with ultrasound.
Limits
- Naegele’s rule adds 280 days and assumes a 28-day cycle with ovulation on day 14. Irregular cycles and ultrasound dates differ.
- Not a diagnosis and not a substitute for prenatal care.
How to calculate
Enter the first day of the last menstrual period. The calculator adds 280 days for the estimated due date and also shows a conception estimate and trimester boundaries.
Due date = LMP + 280 days
Conception estimate = LMP + 14 days
When to use it
- A first planning date before a clinical estimate
Examples
LMP on 1 January
The estimated due date is 8 October in a common year (280 days later).
Offsets from the last menstrual period (LMP)
| Event | Days after LMP |
|---|---|
| Conception estimate | 14 |
| End of first trimester | 90 |
| End of second trimester | 188 |
| Estimated due date | 280 |
FAQ
Is this the same as an ultrasound due date?
Not necessarily. Ultrasound dating can shift the estimate, especially early in pregnancy. Use the date your clinician gives you for care decisions.
What if my cycles are not 28 days?
Naegele’s rule assumes a 28-day cycle with ovulation around day 14. Irregular cycles make the LMP method less reliable.