The 18th week bill includes a new definition for when pregnancy begins. A sentence in the bill defines gestational age as "calculated from the first day of the last menstrual period of the pregnant woman," which would move the beginning of a pregnancy up two weeks prior to conception. The bill's passage would give Arizona the earliest cutoff for late-term abortions in the country; most states use 20 weeks as a definition.
But while the bill's definition of when pregnancy begins is new in legislation, it's not necessarily new for doctors. Elizabeth Nash, states issues manager for Guttmacher Institute, a reproductive health research organization in Washington, said it corresponds with how doctors typically determine gestational age. She said since the exact date of conception cannot be pinpointed, doctors use the day of the woman's last menstrual period to gauge the duration of a pregnancy. The method does not provide an exact date.
