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JACKSON, Miss. — Mississippi's governor signed a law Monday banning most abortions after 15 weeks' gestation, the tightest restrictions in the nation.

Republican Gov. Phil Bryant has frequently said he wants Mississippi to be the "safest place in America for an unborn child."

House Bill 1510's only exceptions are if a fetus has health problems making it "incompatible with life" outside of the womb at full term, or if a pregnant woman's life or a "major bodily function" is threatened by pregnancy. Pregnancies resulting from rape and incest aren't exempted.

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Gov. Phil Bryant outlines his legislative priorities during the State of the State address on Jan. 9, 2018, in House Chambers at the Capitol in Jackson, Mississippi. Rogelio V. Solis / AP file

Mississippi previously tied with North Carolina for the nation's strictest abortion limits at 20 weeks. Both states count pregnancy as beginning on the first day of a woman's previous menstrual period. That means the restrictions kick in about two weeks before those of states whose 20-week bans begin at conception.

The state is bracing for immediate lawsuits. Abortion rights advocates say the law is unconstitutional because it limits abortion before fetuses can live outside the womb. The owner of Mississippi's only abortion clinic in Jackson opposes the law and has pledged to sue.

"We certainly think this bill is unconstitutional," said Katherine Klein, equality advocacy coordinator for the American Civil Liberties Union of Mississippi. "The 15-week marker has no bearing in science. It's just completely unfounded and a court has never upheld anything under the 20-week viability marker."

Dead razor grass and masked fencing at the entrance to the Jackson Women's Health Organization clinic in Jackson, Mississippi, on March 8, 2018. The facility is currently the state's only abortion clinic. Rogelio V. Solis / AP

The bill was drafted with the assistance of conservative groups including the Mississippi Center for Public Policy and the Alliance Defending Freedom.

"We're thrilled that Mississippi lawmakers are taking a step to protect the basic right to life, as well as protecting maternal health," said Jameson Taylor, acting president of the Mississippi Center for Public Policy.

Both Republican-controlled chambers passed the bill overwhelmingly in early March, by a vote of 35-14 in the Senate and 76-34 in the House.

The U.S. Senate failed to pass a 20-week abortion ban bill in January. With 60 "yes" votes required to advance, the bill failed on a 51-46 vote.