In order to compute the gestational age of their fetus by determining the time of last menstrual period. Gestational age is also called the menstrual age which measures the age of the fetus. Gestational age is usually being computed by the date of the last menstrual period of the mother and by doing an obstetric ultrasound to verify the size of the fetus and thru this it can estimate the gestation age of the fetus.