The due date is also calculated on the basis of the last menstrual period if the period is delayed by seven or eight days. The due date is calculated by adding 9 or subtracting 3 to the month and adding 7 to the day of your last menstrual period. However, if your period is usually delayed, your due date may be pushed back, so you can have an ultrasound early in your pregnancy to correct your due date. Early pregnancy ultrasound can estimate the gestational age based on the gestational sac as well as the size of the embryonic bud and the primitive heart tube pulsation. Measuring the fetal head-butt diameter at 11-13+6 weeks of pregnancy is the most accurate way to estimate gestational age. If the difference between the corrected due date and the estimated due date from the last menstrual period is less than one week, the last menstrual period is used. If the difference is more than one week, the corrected due date will take precedence. Pregnant women whose periods are usually delayed and who are not in labor for more than a week after the due date should go to the hospital for checkups and follow the advice of their specialists.