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.

It is true that Arizona now has the most most restrictive state bill out there, but that's because until now other states haven't used the standard medical convention to calculate age. That doesn't mean that it's a good bill, however. As Gunter also points out, the 20-week number is based on the false claim that a fetus can feel pain at 20 weeks. That's also too early to detect the type of diseases and fetal problems that might actually lead to a terminated pregnancy. The Arizona bill also has other onerous and unnecessary provisions, like mandated ultrasounds, heavy new restrictions on non-surgical abortions, and a clause stating as fact that "abortion can cause serious both short-term and long-term physical and psychological complications for women." All of those provisions are more harmful to women's choice, but the fuss over the gestational age rule has obscured them. There are plenty of reasons for pro-choice activists to hate this new law, but that calculation shouldn't be one of them.