Due Date Calculator: Understanding Naegele's Rule
How pregnancy due dates are calculated, what Naegele's Rule means, and tracking your pregnancy timeline.
Naegele's Rule, developed in the early 1800s, remains the standard method for calculating estimated due dates. It adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). Our Due Date Calculator supports both LMP and conception date inputs.
Why 280 Days?
The average pregnancy lasts 280 days from the LMP, or 266 days from conception. However, only about 4% of babies are born on their due date. Most births occur within a 2-week window around the due date.
Pregnancy Health Tools
During pregnancy, nutrition becomes critical. Use our Calorie Calculator to understand increased calorie needs (typically 300-500 extra calories per day in the second and third trimesters). Protein intake should increase to support fetal growth. Stay well-hydrated using the Water Intake Calculator.
Note that BMI interpretation changes during pregnancy — weight gain is expected and healthy within recommended ranges.
Try the Calculator
Put these concepts into practice with our free Due Date Calculator. No signup required.