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RENEE MONTAGNE, HOST:

This is MORNING EDITION from NPR News. Good morning. I'm Renee Montagne.

DAVID GREENE, HOST:

And I'm David Greene. Banning abortions after a specific point in pregnancy has been a trend in the states this year. Last week, Texas became the 12th state to ban most abortions after 20 weeks. Now, if you've been following the debate, you might be confused about just what that 20 weeks refers to. And how it relates to the question of whether these laws might pass constitutional muster. We asked NPR's Julie Rovner to help sort this out.

JULIE ROVNER, BYLINE: Like all but one of the abortion bans passed so far in the state, the Texas law would ban nearly all abortions later than 20 weeks after fertilization. But that's not the same as saying 20 weeks of pregnancy. Because that's not how doctors measure pregnancy.

DANIEL GROSSMAN: When we refer to the weeks of pregnancy or the weeks of gestation, we measure pregnancy from the date of the last normal menstrual period.

ROVNER: Daniel Grossman is an assistant professor of obstetrics, gynecology and reproductive sciences at the University of California, San Francisco.

GROSSMAN: For a woman who has a normal menstrual period, ovulation or fertilization would generally occur actually two weeks later, after that start of that normal menstrual period. So if we wanted to actually measure, the age of the embryo or the fetus, is essentially two weeks less than the number of weeks measured from the last menstrual period.

ROVNER: 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? Grossman says it's mostly a habit that's never been updated.

GROSSMAN: Especially, you know, in the old days before ultrasound was widely available and widely used, the last menstrual period was something that was knowable and was measurable, whereas it wasn't always known exactly when fertilization took place.

ROVNER: 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, says Elizabeth Nash.

ELIZABETH NASH: What we're seeing with these laws is that they are pegging the beginning of pregnancy to fertilization.

ROVNER: Nash tracks state issues for the Guttmacher Institute, an abortion rights think tank.

NASH: So when we're talking 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.

ROVNER: So why, then do people keep referring to these as 20 week laws?

NASH: The reason people use this 20 weeks as the description for these laws is that 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.

ROVNER: But whether the laws seek to ban abortion at 20 weeks or 22 weeks, one thing is clear, says OB/GYN Daniel Grossman. The ban they would impose is earlier than what's currently considered viability, or when a fetus can survive outside the womb.

GROSSMAN: I think there's definitely consensus that viability doesn't happen before 24 menstrual weeks. 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 is a concern.

ROVNER: 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. Here's Douglas Johnson of the National Right to Life Committee at a news conference on the federal version of the 20 week ban, talking about an earlier ban that few thought would survive court scrutiny.

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DOUGLAS JOHNSON: The Partial Birth Abortion Ban Act was struck down by every lower federal court that considered it. Three U.S. District courts, three separate 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.

ROVNER: So whether you count to 20 or 22, the ultimate number that will matter is five - the number of Supreme Court justices needed for a majority. Julie Rovner, NPR News, Washington.

(SOUNDBITE OF MUSIC)

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