Missouri doctors and hospitals will face new restrictions and penalties for performing late-term abortions after Gov. Jay Nixon announced Thursday that he would let legislation backed by anti-abortion groups take effect without his signature.

The law is part of a trend among states to limit abortions past the point when a fetus may be able to live outside the womb. Missouri's proposal leaves it to doctors to determine viability on a case-by-case basis, unlike new laws in several other states that bar most abortions after 20 weeks.

Nixon, a Democrat, cited a section of the Missouri Constitution that allows bills to become law if not signed or vetoed by the governor within 45 days after the Legislature adjourns. Thursday was the deadline for him to take action.

"This legislation was approved by an overwhelming, bipartisan majority in both houses," Nixon said in a brief written statement.

At a signing ceremony in Kansas City for unrelated legislation, Nixon told reporters that abortion "is a public policy obviously that's talked about in Missouri tremendously, and I've tried to _ you know _ make sure that we are sensitive to all sides of the issue."

This marks the second straight year that Nixon has allowed an anti-abortion bill to become law without his signature as he attempts to walk a political tightrope by neither directly supporting nor opposing the measures. His decision prompted praise from some anti-abortion activists and disappointment among abortion-rights backers.

Since 1974, Missouri law has prohibited aborting viable fetuses unless necessary to preserve the life or health of the woman. The new law will delete that general health exception effective Aug. 28. Instead it will allow such abortions only to save the woman's life or when the pregnancy poses a serious risk of permanent physical harm to a major bodily function.

The effect could be to eliminate a woman's mental health as a justifiable reason for a late-term abortion _ a change which some abortion-rights advocates say could run afoul of previous U.S. Supreme Court rulings.

Missouri is among 31 states that currently have a general health and life exception to their bans on late-term abortions, according to the New York-based Guttmacher Institute, a research group that supports abortion rights. With its new law, Missouri will join Idaho, Indiana, Kansas and Nebraska as now granting exceptions only for a woman's physical health or life.

"It is an effort to try to restrict abortion as much as possible, and one way we are seeing this play out is by narrowing these health exceptions," said Elizabeth Nash, a policy analyst for the institute.

Under Missouri's new law, doctors who abort a viable fetus when a woman does not qualify for an exception could face prison sentences of up to seven years, fines up to $50,000 and the loss of their medical licenses. Hospitals and surgical centers allowing such abortions also could lose their state licenses.

But those penalties may seldom come into play. The state health department says just 63 of the 6,881 abortions recorded in Missouri in 2009 were on fetuses at least 21 weeks old, and none were reported as being viable. Figures were not available for 2010.

Some anti-abortion lawmakers and activists have questioned the veracity of those figures, suggesting that it is in the best interest of abortion providers not to report that a late-term abortion was conducted on a viable fetus.

Missouri law already requires doctors to determine the viability of a fetus when a woman is at least 20 weeks pregnant and requires that a second physician be present for abortions of viable fetuses. The new law will require that a second physician concur the abortion is necessary to preserve the life of the woman or prevent permanent harm to one of her major bodily functions.

The result is "it will save babies and it will protect women from the effects of late-term abortions," said Susan Klein, a lobbyist for Missouri Right to Life. "When you're five months into pregnancy _ at this point and time _ you could feasibly save the life of the baby and save the life of the mother."

Some abortion rights groups had called upon Nixon to veto the bill.

Under the new late-term abortion law, "Missouri's emergency exception is one of the narrowest in the country _ it's basically your kidneys are going to fail or you're going to die," said Pamela Sumners, executive director of NARAL Pro-Choice Missouri.

The bill passed both the House and Senate with bipartisan, two-thirds majorities that could have been sufficient to override a gubernatorial veto. But the rhetoric between supporters and opponents was more intense in the House.

House Speaker Steven Tilley, R-Perryville, said Thursday that "late-term abortions not only are wrong, they're inhumane." He added: "When you've got a fetus that's viable to live, I don't see how anyone can argue that baby should be aborted."

The law that Nixon allowed to take effect last year requires abortion providers to offer women the chance to view an ultrasound and listen to the heartbeat of their fetuses. Among other things, it also requires a consultation before an abortion to occur in person, instead of over the phone.

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Associated Press writer Maria Sudekum Fisher in Kansas City contributed to this report.