Banning abortions after a specific point in pregnancy has been a popular trend in the states this year. Last week, GOP Gov. Rick Perry made Texas the 12th state to ban most abortions after 20 weeks.
But how states define the starting point for that 20 weeks may cause headaches for women and their doctors and ultimately affect whether these laws pass constitutional muster.
Like all but one of the abortion bans passed so far in the states, the Texas law starts its 20-week calendar at fertilization. But that's not the same as saying 20 weeks of pregnancy, because that's not how doctors measure pregnancy.
"When we refer to the weeks of pregnancy, weeks of gestation, we measure pregnancy from the date of the last normal menstrual period," says Dr. Daniel Grossman. He's an assistant professor of obstetrics, gynecology and reproductive sciences at the University of California, San Francisco and a vice president of IBIS Reproductive Health, a reproductive rights advocacy group.
"For a woman who has a normal menstrual period, ovulation or fertilization would generally occur two weeks later, after that start of that normal menstrual period," Grossman says. "The age of the embryo or the fetus is essentially two weeks less than the number of weeks measured from the last menstrual period."
Last menstrual period, or LMP, is generally how doctors refer to the weeks of pregnancy. Forty weeks LMP is considered full term for a normal pregnancy, even though at that point fertilization occurred only 38 weeks before. So why do doctors use a measurement that's so imprecise?
That standard developed in the old days before ultrasound was widely used, Grossman says. "The last menstrual period was something that was knowable and was measurable, whereas it wasn't always known when fertilization took place."
With few exceptions, however, that's not how the state laws and a bill that passed the U.S. House last month are being written.
"What we're seeing with these laws is that they are pegging the beginning of pregnancy to fertilization," says Elizabeth Nash, who tracks state issues for the Guttmacher Institute, an abortion rights think tank.
"So when we talk about a law that bans abortion at 20 weeks post-fertilization, we're really talking about a law that bans abortion at 22 weeks of pregnancy," she says.
Why is it, then that people keep referring to these as 20 week laws?
Nash says it's not that hard to figure out. "That's the term that is used in the bill, and oftentimes when you see a term used in the bill it becomes the headline," she says.
But whether the laws seek to ban abortion at 20 weeks or 22 weeks, one thing is clear, says Daniel Grossman. The ban they would impose is earlier than what's currently considered viability, or when a fetus can survive outside the womb.
"I think there's definitely consensus that viability doesn't happen before 24 menstrual weeks," he says. "So when we're talking about banning abortion at 20 or 22 weeks even, that's clearly at least two weeks before the earliest point in pregnancy where viability would be a concern."
That's important, because current Supreme Court precedent says states can't ban abortion before viability. But those pushing these laws clearly hope that by the time one of these laws makes its way to the justices, they might change their minds.
"The Partial Birth Abortion Ban Act was struck down by every lower fed court that considered it," Johnson said last week at a press conference on the federal version of the 20-week ban. "Three U.S. District Courts; three U.S. Courts of Appeals all ruled it was in clear violation of U.S. Supreme Court precedent. But when it reached the U.S. Supreme Court they said otherwise. And they upheld it."
So whether you count to 20 or 22, the ultimate number that will matter most is five the number of Supreme Court justices needed for a majority.