Gestational length is 266 days from conception so for the average person, the easiest way to figure out the due date if one has a 28-day menstrual cycle is to take the date of the last menstrual period, go back three months and then add a week. For those of us in obstetrics, we usually rely on this wheel that allows us to calculate it out week to week and frequently bring this out and show it to the patient. What has become the most accurate way to assess gestational age is ultrasound and in most pregnancies these days, people undergo ultrasounds on the first trimester that allows the gestational age to be precisely calculated.