EDD, EDC and the abbreviations used in pregnancy notes
| Abbreviation | Meaning |
|---|---|
| EDD | Estimated (or expected) date of delivery — the due date |
| EDC | Estimated date of confinement — an older term for the same date |
| LMP | First day of the last menstrual period |
| GA | Gestational age, in weeks + days from the LMP |
| CRL | Crown–rump length, the first-trimester ultrasound measurement used for dating |
How the EDD is calculated
- EDD by LMP: LMP + 280 days (Naegele’s rule), adjusted for cycle length.
- EDD by conception: conception date + 266 days.
- EDD by ultrasound: scan date + (280 − gestational age on the scan in days).
The result shows the EDD/EDC, the equivalent LMP, the gestational age today and the estimated conception date, followed by Naegele’s rule step by step so the calculation can be checked. For the expected date of delivery formula in more detail, see the Naegele’s Rule Calculator.
Which EDD should be used?
Professional guidance (for example from the American College of Obstetricians and Gynecologists) recommends confirming the EDD with the earliest reliable ultrasound and changing it only if the difference from the LMP date exceeds set thresholds. Once confirmed, the EDD should be documented and not changed later. The Accurate Due Date Calculator lists those thresholds.