How the Due Date Is Calculated
The estimated due date (EDD) is most commonly found using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle is longer or shorter than 28 days, the date shifts by the difference.
Due Date = LMP + 280 days + (Cycle Length − 28)
The Three Trimesters
| Trimester | Weeks |
|---|---|
| First | 1 – 13 |
| Second | 14 – 27 |
| Third | 28 – birth |
Important Note
Only about 1 in 20 babies arrive on the exact due date — most are born within two weeks either side. This calculator is for general guidance and does not replace medical advice. An ultrasound dating scan provides a more accurate estimate.
Frequently Asked Questions
What if I don't know my LMP?
If you're unsure of your last period, an early ultrasound is the best way to date the pregnancy.
Can I use this with irregular cycles?
You can adjust the cycle length, but with very irregular cycles the LMP method is less reliable — rely on your doctor's scan.