Identify the first day of the last menstrual period
Add 280 days, or 40 weeks, from that date
Use Naegele’s rule: subtract 3 months, add 7 days, and add 1 year
Confirm the date with an early ultrasound if available
Adjust the due date if menstrual cycles are irregular or if conception date is known
Use clinical evaluation and prenatal records to verify the estimated due date