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.”