A pregnant woman protests abortion at the Supreme Court in 2008Kevin Lamarque / Reuters

In practice, the number is small. Approximately 9,090 women in the United States had abortions after their 21st week of pregnancy in 2012. That’s 1.3 percent of all abortions, and roughly 0.14 percent of all pregnancies, based on the 2010 U.S. pregnancy rate.

Yet states keep creating legislation on this issue, proposing abortion bans at 24 or 22 weeks. Many—like South Carolina, where one such bill was signed into law last week—provide exceptions for medical emergencies or fetal anomalies. In fact, many of the women who seek abortions at this stage in their pregnancies do so for health reasons, so these bans affect only a subset of those 9,090 women. Moreover, in states like South Carolina, it can be difficult or even impossible to find an abortion provider who will perform the procedure at that stage. The Planned Parenthood in Columbia, for example, will only provide abortions up until 13 weeks and 6 days into the pregnancy.

So it might seem surprising that these laws, which affect very few women, keep getting pushed and passed. But even though laws like these are often contested through protests, lawsuits, or both, they’re not that different from the abortion prohibitions in a vast majority of states around the country. There’s a kind of national mean on abortion limits in the United States—a significant majority of states, from the most conservative to the most liberal, ban the procedure somewhere between 22 weeks and viability, which “most obstetrician-gynecologists understand ... as occurring near 24 weeks gestation,” according to American Congress of Obstetricians and Gynecologists. There are only seven states—Alaska, Colorado, New Hampshire, New Jersey, New Mexico, Oregon, and Vermont—that don’t limit abortion by gestational age at all. And only three let women wait until their third trimester begins, around 28 weeks. State by state, a nearly uniform consensus has emerged in America: After roughly two dozen weeks, women should not be able to get an abortion for non-medical reasons.

The reasons behind these bans differ from state to state, though. That’s what this legislation mostly seems to be about: the principles, rather than the practicalities, of abortion. Most of the newer limits on gestational age have to do with fetal pain—legislators claim they’re prohibiting the abortion of fetuses that can feel physical distress. But confusingly, these legislators often count the number of weeks in a pregnancy differently than doctors do. Although states like South Carolina claim to have passed a 20-week abortion ban, in medical terms, that’s actually 22 weeks.

Here’s why: Doctors measure gestational age by a standard called LMP, which dates a pregnancy from the woman’s last menstrual cycle. But starting with a 2010 Nebraska bill, some legislators started using the number of weeks “post-fertilization” as the standard for gestational age—equivalent to two weeks after a woman’s period, the average time when most women ovulate. Some states have tried to pass bans that would apply even earlier in a pregnancy, like Arizona’s 2012 law that tried to set the limit at 18 weeks post-fertilization or 20 weeks after a woman’s last period. That measure was struck down by the courts, though. For now, bans that apply 20 weeks post-fertilization and 22 weeks LMP are about as low as states can go. More than a dozen have these limits in place.

This linguistic sleight-of-hand might be one way legislators are trying to create a sense of fetal personhood, said Elizabeth Nash, a staffer at the Guttmacher Institute, a pro-choice research organization. “There’s been an effort by conservative lawmakers to peg the beginning of pregnancy to fertilization,” she said. “They’ve talked about personhood starting at fertilization for a long time, and this is a way to codify that and get it into law.”

Although laws like South Carolina’s aren’t effectively that different than those in other states, Nash said, they shift the standard set by the Supreme Court in Roe v. Wade and the 1992 case Planned Parenthood v. Casey. If measures of fetal pain were to become the standard, “that would really upend the underpinnings of abortion rights: a state cannot ban abortion before viability,” she said.

The standard of viability presents its own challenges, though. “I don’t find the concept of viability to be a particularly helpful one for physicians,” said Edward Bell, a doctor at the University of Iowa who studies premature infants. “In a practical sense, when doctors talk about viability, they mean, ‘At what point in pregnancy is it reasonable to try and resuscitate a baby and try and offer treatment?’” That calculation might depend on a number of factors, from the gender of the fetus to the kind of neonatal care the mother has received, Bell said. “It’s rather simplistic to reduce the conversation to the number of completed weeks in pregnancy.”

After Bell co-published a paper on variability in hospital care for very premature infants last year, “our paper was cited on the floor of Congress by the anti-abortion lobby as evidence that abortion should be restricted at an even earlier time in pregnancy,” he said. A lot of medical research focuses on the chances of survival for infants born at a certain number of weeks—chances which have been getting higher and higher for babies born earlier and earlier into pregnancies. But that work is not designed to apply to abortion. It’s aimed to address concrete situations in which parents and doctors are trying to make medical choices, not the kind of theoretical point of viability most abortion laws invoke. “It’s unfortunate that the research is used this way,” Bell said.

Ultimately, there is no perfect way of numbering the days into a pregnancy, and no set way of knowing how soon a fetus could survive outside of the womb. The law doesn’t accommodate this ambiguity very well, though. Instead, it offers a misleading sense of  authority or certainty, allegedly based on firm knowledge of what’s happening to a fetus at any given point in its gestation.

Especially with late-term pregnancy bans, these laws can seem more symbolic than practical—crafted with legal standards and political capital in mind, rather than tangible outcomes for women. Georgia’s 20-week post-fertilization / 22-week LMP ban became law in 2012, but a court put it on hold. Last October, a judge lifted that hold, but due to a clerical error, no government department or party in the original lawsuit was notified.

The new ban went into effect. No one noticed.

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