in the medical profession Gestational age has always been counted from last menstrual period. When people are 20 weeks pregnant they are in fact only 18 weeks pregnant, and a normal 40 week gestation is really only 38 weeks long. So the definition Arizona is using is exactly how we calculate in medicine. Pakman's semantics on this are silly. Pregnancy is not defined by a woman's menses, it is defined by a positive test for pregnancy, the gestational age is defined by her menses. Even ultrasound dates are base on that calculation. When an ultrasonographer states that a fetus is 8 weeks 4 days, +/- 2 days. it really means that the actual fetus is roughly 2 weeks less that that.
time from conception date is not the same as gestational age. They are approximately 2 weeks apart