Use the first day of the last menstrual period (LMP)
Add 280 days, or 40 weeks, to the LMP date
Use Naegele’s rule: add 1 year, subtract 3 months, add 7 days
If the menstrual cycle is longer or shorter than 28 days, adjust the date accordingly
Use an early ultrasound for the most accurate estimate if LMP is uncertain
Confirm the date with a healthcare provider for medical accuracy