How Is a Due Date Calculated?
This calculator uses Naegele's rule, the standard method used by healthcare providers worldwide. It estimates your due date as 280 days (40 weeks) from the first day of your last menstrual period (LMP), adjusted for your cycle length.
Why Adjust for Cycle Length?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation likely occurs on a different day. The calculator adjusts by adding or subtracting the difference from 28 days, giving a more personalized estimate.
How Accurate Is a Due Date?
Only about 5% of babies are born on their exact due date. Most births occur within a window of 2 weeks before or after. Your healthcare provider may adjust your due date based on ultrasound measurements, especially if performed in the first trimester when dating is most accurate.
Key Milestones
- Week 12: End of first trimester, major organ formation complete.
- Week 20: Anatomy scan ultrasound.
- Week 37: Considered "early term", baby is ready for life outside the womb.
- Week 40: Your estimated due date.