What “estimated due date” means
EDD stands for estimated date of delivery or estimated due date. It is the date a pregnancy reaches 40 weeks, counted from the first day of the last menstrual period. It is used to plan prenatal care, time screening tests and judge whether a baby is growing as expected — but it is a statistical midpoint rather than a prediction of the actual birthday.
Naegele’s rule, step by step
The calculation behind every EDD is short:
- Find the first day of your last menstrual period (LMP).
- Add 280 days.
- If your cycle is regularly longer or shorter than 28 days, add or subtract the difference.
The shortcut taught to midwives is LMP + 7 days − 3 months + 1 year. For an LMP of January 10: + 7 days = January 17, − 3 months = October 17, and the year moves forward by one. Counting exact days gives October 17 as well in this case, but around short months the two approaches can disagree by a day, so the calculator counts days.
Why cycle length matters
Naegele’s rule assumes ovulation on day 14 of a 28-day cycle. The phase after ovulation (the luteal phase) is fairly constant at about 14 days for most people, so a longer cycle usually means later ovulation — and a later due date. The calculator applies this adjustment automatically:
| Average cycle length | Adjustment to the due date |
|---|---|
| 24 days | 4 days earlier |
| 26 days | 2 days earlier |
| 28 days | no change |
| 30 days | 2 days later |
| 32 days | 4 days later |
| 35 days | 7 days later |
If your cycles vary a lot from month to month, the adjustment is less reliable and an early ultrasound becomes more important for dating. You can enter scan results in the Accurate Due Date Calculator.
Estimated, expected or EDC — what is the difference?
None. You may see the same date called several names: estimated due date, expected due date, EDD (estimated or expected date of delivery) and, in older notes, EDC (estimated date of confinement). They all mean the date the pregnancy reaches 40 weeks, so an expected due date calculator and an estimated due date calculator do exactly the same job.
A worked example with a long cycle
Suppose your last period started on March 3 and your cycles are usually 33 days long:
- Standard due date: March 3 + 280 days = December 8.
- Your cycle is 5 days longer than 28, so ovulation probably happened about 5 days later than day 14.
- Adjusted estimated due date: December 8 + 5 days = December 13.
Enter the same dates above and the calculator gives the same answer, together with your current week of pregnancy and the days remaining. If your cycles vary by more than a few days from month to month, use the average and expect an early ultrasound to refine the date.
When the EDD is set by a scan instead
If your first ultrasound measures the pregnancy noticeably earlier or later than your period suggests, your provider may move your EDD to match the scan. After that, every “weeks pregnant” figure is counted from the new date. You can check the updated dates with the Reverse Due Date Calculator or enter the scan result directly in the Accurate Due Date Calculator.
EDD versus the date your baby arrives
Most births happen within roughly two weeks of the EDD, and only a small share — often quoted as around 4–5% — on the day itself. First babies are, on average, slightly more likely to arrive after the EDD than before it. Your EDD is still extremely useful: every “weeks pregnant” figure your care team uses is counted back from it. To see that number for today, use the Pregnancy Week Calculator, or compare the LMP method with other methods on the main Due Date Calculator.