Washington (AFP) - Two months after refusing to intervene, the US Supreme Court is to hear challenges on Monday to a Texas law that bans abortions after six weeks of pregnancy and makes no exceptions for rape or incest.

The nine-member court, which includes six conservative justices, will listen to two hours of arguments by parties in a closely-watched case with far-reaching human and political ramifications.

Texas, the country's second-largest state, is being sued by Democratic President Joe Biden's Justice Department and a coalition of abortion providers, accused of enacting abortion restrictions they say are "plainly unconstitutional."

Texas Senate Bill 8 (SB8) bans abortions after a heartbeat can be detected in the womb, which is normally around six weeks, when many women do not even know they are pregnant.

Laws restricting abortion have been passed in other Republican-led states but were struck down by the courts because they violated previous Supreme Court rulings which guaranteed the right to an abortion until the fetus is viable outside the womb, which is typically around 22 to 24 weeks.

The "Texas Heartbeat Act" differs from other efforts in that it insulates the state by giving members of the public the right to sue doctors who perform abortions, or anyone who helps facilitate them, once a heartbeat is detected.

They can be rewarded with $10,000 for initiating cases that land in court, prompting criticism that the law encourages people to act as vigilantes.

The framing of the Texas law has complicated the intervention of the Justice Department because of a principle called "sovereign immunity," said Mary Ziegler, a professor of constitutional law at Florida State University and visiting professor at Harvard University.

"The Eleventh Amendment of the US Constitution limits the circumstances under which states can be sued," Ziegler told AFP.

"The Supreme Court has carved out an exception that allows plaintiffs to sue to get an injunction when an official is enforcing a potential unconstitutional law," she said.

"Texas says that under SB8, no official is theoretically allowed to enforce the law," Ziegler said. "Texas has successfully argued to date that it is immune from suit."

'Why are they making me keep it?'

The Supreme Court was asked by abortion providers to block the Texas law when it took effect on September 1 but the court declined to do so citing "procedural issues."

Biden was among those who criticized the court for failing to tackle a law that "blatantly violates the constitutional right established under Roe v. Wade," the landmark 1973 Supreme Court ruling that enshrined a woman's legal right to an abortion. 

On the ground, clinics in Texas, fearful of potential ruinous lawsuits, closed their doors and the number of abortions in the state fell to 2,100 in September from 4,300 a year earlier, according to a University of Texas study.

Planned Parenthood, one of the largest providers of women's health care in the nation, sent a 30-page legal brief to the court containing testimony from women and doctors affected by the Texas law.

A 16-year-old Texas girl identified as F.P. said she is not ready to have a baby but may not be able to afford to go to another state for an abortion.

A single mother identified as D.O. said she is seeking an abortion after she "finally got away" from an abusive relationship.

One patient, identified as I.O., was 12 years old.

"The mother said they could not travel out of State -- they had barely made it to the Texas health center," the brief said. The 12-year-old was quoted as saying, "Mom, it was an accident. Why are they making me keep it?"

Clinic staff recounted crying with patients after telling them they would be unable to provide abortions because of the new law.

'Is there a fifth vote?'

The Supreme Court could make a decision at any time after oral arguments but is widely expected to rule before hearing another abortion case on December 1.

In that case, the court will hear a challenge to a Mississippi law that bans abortion after 15 weeks.

At least four justices appear ready to block the Texas law: the three liberals on the court and Chief Justice John Roberts, who expressed concerns about SB8 when it previously appeared before the court.

"Now the question is 'Is there a fifth vote?'" Steve Vladeck, a law professor at the University of Texas, asked on a podcast.

Abortion providers are cautiously optimistic.

"We are hopeful the court will step in and block SB8 from continuing to wreak havoc," said Alexis McGill Johnson, president and CEO of Planned Parenthood.

Vladeck said he saw the court potentially "splitting the difference."

"They could strike down the Texas law but allow the Mississippi law to remain on the books," he said. "And then everyone's pissed off and the court can say 'Look we're not partisan.'"