How the due date is estimated
This calculator uses Naegele's rule: your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period. It assumes a regular 28-day cycle with ovulation around day 14.
Pregnancy stages
- First trimester: weeks 1–13
- Second trimester: weeks 14–27
- Third trimester: weeks 28 until birth
When the estimate changes
If your cycle is longer or shorter than 28 days, or if an early ultrasound shows a different gestational age, your doctor may adjust the due date. Most babies are born within two weeks before or after the estimate.
Medical note
This tool is informational only and does not replace prenatal care. Always follow your healthcare provider's guidance.
Frequently asked questions
How accurate is the due date?
It is an estimate. Only a small percentage of babies are born exactly on the due date.
What if my cycle is irregular?
Your doctor may use an ultrasound to date the pregnancy more accurately.
Is my date stored?
No. Everything stays in your browser.