How the Due Date Calculator Works
This calculator uses Naegele's rule, the standard method in obstetrics for determining the estimated due date (EDD). This method is recommended by ACOG (American College of Obstetricians and Gynecologists) and is used by healthcare providers worldwide.
Naegele's Rule
The rule is simple: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This calculator also includes a cycle length correction: if your cycle is longer or shorter than 28 days, the date is adjusted accordingly.
Corrected formula: EDD = Last menstrual period date + 280 days + (cycle length - 28)
For example, if your last period started on January 1, 2026 with a regular 28-day cycle, the estimated due date would be October 8, 2026.
Why Counting Starts from the Last Period
The count begins from the last menstrual period because it is a certain and easily remembered date. Conception typically occurs about 14 days after the start of the cycle (at ovulation), but the exact date is rarely known. For this reason, the first two weeks of "pregnancy" actually precede conception.
When an Ultrasound Date Is More Reliable
In some cases, the doctor may prefer ultrasound dating over Naegele's rule:
- When the menstrual cycle is irregular or the LMP date is uncertain
- When the first-trimester ultrasound shows a discrepancy greater than 7 days from the menstrual dating
- In cases of assisted conception (IVF/ART), where the embryo transfer date is known
The CRL (Crown-Rump Length) measurement in the first trimester is the most precise ultrasound parameter, with a margin of error of only plus or minus 5 days.
Accuracy of the Due Date
Remember that the estimated due date is a statistical estimate: only about 5% of babies are born on the exact predicted date. A full-term delivery is considered normal between weeks 37 and 42. Always consult your obstetrician for definitive dating and pregnancy monitoring.