A normal pregnancy is dated based on a 40 week calendar used by a physician to determine length of pregnancy and the age of the embryo/fetus. This calendar is calculated based on the first day of the last menstrual period, not on the day of conception. For example, if a woman is 6 weeks into a pregnancy, the embryo was conceived during week 2, not week 0.
However, the calendar was never meant to be used as a basis for the law that was signed by Gov. Brewer.