Pregnancy Due Date Calculator
Calculate your estimated pregnancy due date from your last menstrual period using Naegele's rule, adjusted for cycle length.
✓ Runs in your browser · Updated 2026-03-31Enter values and click Calculate Due Date to see results
What is Naegele's Rule?
Naegele's rule is the standard method for estimating the due date of pregnancy. It calculates the estimated date of delivery (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP), with an adjustment for cycle length.
EDD = LMP + 280 days + (Cycle Length − 28) days
Standard cycle = 28 days. Longer cycles push the due date later; shorter cycles bring it earlier.
Understanding Trimesters
First Trimester (Weeks 1–12): Major organ development occurs. Common symptoms include nausea, fatigue, and breast tenderness.
Second Trimester (Weeks 13–26): Often considered the most comfortable period. The baby's movements become noticeable.
Third Trimester (Weeks 27–40): Rapid growth and weight gain. The body prepares for labour and delivery.
Important Notes
This calculator provides an estimate only. Only about 5% of babies are born on their exact due date. Most deliveries occur within 2 weeks before or after the estimated date. Always consult your healthcare provider for accurate dating, especially through ultrasound confirmation.
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Frequently Asked Questions
How is the due date calculated?
Using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle. Our calculator also allows cycle length adjustment.
How accurate is the due date?
Only about 4-5% of babies are born on their exact due date. Most deliveries occur within 2 weeks before or after the EDD. The due date is an estimate, not a guarantee.