health ToolUpdated 2026-09-19
Pregnancy Due Date Calculator
Estimate your baby’s due date based on the first day of your last menstrual period (LMP) using standard clinical Naegele’s Rule.
Important Medical Disclaimer: This health tool is designed solely for informational, screening, and educational purposes. It does not provide medical diagnosis, treatment, or individualized clinical advice. Always consult a qualified physician or healthcare provider regarding any health condition.
Formula & Mathematical Methodology
Naegele’s Rule: LMP + 280 Days (40 Weeks)
Adds 280 days (40 completed weeks) to the first day of the last menstrual period, adjusted for cycle duration.
How to Use This Pregnancy Due Date Calculator
- Select the date of your last period and your typical menstrual cycle length.
Step-by-Step Worked Example
Scenario: LMP on January 1st with 28-day cycle
1. Add 1 year, subtract 3 months, add 7 days: October 8th
2. Total gestational duration: 40 weeks (280 days)
Final Calculated Result: October 8th (Estimated Due Date)
What Does Your Result Mean?
Only about 4% to 5% of babies are born on their exact due date; delivery between weeks 37 and 42 is considered full term.
Common Practical Applications:
- Trimester planning
- Doctor appointment scheduling
- Prenatal preparation
Important Limitations & Assumptions:
- An ultrasound scan performed by an obstetrician provides the most authoritative clinical dating.
Frequently Asked Questions
What is Naegele’s Rule?
A standard medical formula established by German obstetrician Franz Naegele in the 19th century to calculate estimated delivery dates.
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