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Gov. Mike Beebe, a Democrat, had vetoed the 20-week limit on Tuesday, saying it was likely to be found unconstitutional, but the newly Republican-controlled Senate voted to override Mr. Beebe's veto on Thursday; the House had already done so Wednesday. The measure is set to take effect immediately.

Arkansas is the 10th state to outlaw abortions after 20 weeks, in part based on the theory that fetuses can feel pain at that stage, a notion disputed by mainstream medical associations.

The 20-week limit also violates the legal threshold set by the Supreme Court, which has held that states cannot ban abortions before the fetus becomes viable. Such a limit has not yet been tested by the courts.

Doctors say viability, the ability to survive outside the womb, usually occurs after at least 24 weeks.

"We're seeing a real defiance of what the Supreme Court has held," said Elizabeth Nash, the state issues manager with the Guttmacher Institute, a research group in Washington. "The Supreme Court says viability is determined by a doctor."

Jason Rapert, an Arkansas state senator who sponsored the 12-week limit, says the Supreme Court provides too little guidance on determining viability, but that a heartbeat is an early sign of life. His goal is to prevent what he described as "abortion being used as birth control."

"When there is a heartbeat, there is life," Mr. Rapert said. "We do not need to be killing little babies."

The 12-week limit, which Governor Beebe is also expected to veto if it reaches him, would be an even greater challenge to existing constitutional standards.

The bill, which passed 26 to 8, includes exceptions for rape, incest and medical emergencies.

In a statement, Mr. Beebe said he had not made a decision on the 12-week ban but accepted the Legislature's override of his earlier veto.

"This is the Legislature's prerogative," he said. "It's part of the process, and we all know that. They did what they thought they should do and I did what I thought I should do."

Nine other states have passed 20-week limits since 2010, when Nebraska became the first. In two states, Georgia and Arizona, the laws were blocked by legal challenges.

The American Civil Liberties Union has said it will challenge both of Arkansas's laws. A Planned Parenthood statement called the 12-week ban "blatantly unconstitutional" and "a brazen affront to the needs of women."

Many states, especially in the South, are weighing efforts to limit abortions. In Alabama, the Legislature will vote next week on tighter regulations for abortion clinics. In Mississippi, the state's only abortion clinic is battling new regulations that its leader says could force it to close.

Last year, states across the country passed 43 bills restricting access to an abortion — the second-highest number of such measures ever passed in a single year, according to the Guttmacher Institute.