Start counting from the first day of your last menstrual period
Count each completed 7-day period as one pregnancy week
Add the days since your last period to the weeks already completed
Use the gestational age shown by your doctor or ultrasound if available
Remember that pregnancy is usually dated as 40 weeks from the last menstrual period
Count forward week by week until delivery
If your periods are irregular, use an early ultrasound for a more accurate estimate