Pro-choice supporters are ridiculing Arizona's new restrictive abortion bill, but its most controversial provision isn't actually that controversial if you're an obstetrician.
Yesterday, Governor Jan Brewer signed the new bill into law that bans all abortions after the age of the fetus reaches 20 weeks. It also mandates that the gestational age should be calculated from date of the last menstrual period (or LMP) for the woman who is pregnant. However, given the mechanics of ovulation and fertilization that LMP date is usually about two weeks before the egg is fertilized and the woman actually becomes pregnant. Hence, the new meme that Arizona is declaring women to be pregnant two weeks before they even have sex.
In medical terms, however, what Arizona has mandated is not that unusual or controversial. Jennifer Gunter, is an OB/GYN who has written extensively about abortion and abortion politics. (This story about treating a woman who had a botched abortion is particularly powerful.) She has previously explained on her blog that the age of pregnancy is always calculated from the LMP, because that is the only date in the reproductive cycle that can be determined with any real accuracy. According to Gunter, when a doctor talks about gestational age they are always "starting the clock" with the pregnant woman's last period. Arizona is merely codifying an established medical practice.