Use the first day of the last menstrual period (LMP)
Add 280 days, or 40 weeks, to the first day of the LMP
Use Naegele’s rule: subtract 3 months, add 7 days, and add 1 year to the first day of the LMP
Use an early ultrasound for the most accurate dating if the LMP is unknown or irregular
Use the date of conception if it is known, then add 266 days, or 38 weeks
Confirm the due date with a healthcare provider
Adjust the due date if ultrasound measurements differ significantly from the LMP-based estimate