Washington (AFP) - The US Supreme Court will step into the divisive and emotional battle over the abortion drug mifepristone Wednesday, when it is expected to rule on tough new court-ordered restrictions on the widely used pill.

It sets up the conservative-leaning bench for its most significant intervention on reproductive rights since a seismic ruling 10 months ago that stripped women in America of the constitutional right to a termination.

Since then, anti-abortion groups have refocused their efforts on banning mifepristone, which was approved by the Food and Drug Administration (FDA) back in 2000 and currently accounts for more than half of all abortions in the United States.

Those efforts bore fruit earlier this month when a federal judge said the FDA approval was flawed and the drug should be withdrawn.

An appeals court blocked any ban on the pill, but imposed tough restrictions on access, after which the baton was handed to the Supreme Court which has until midnight Wednesday to step in.

The multi-level rulings and temporary stays have created a climate of legal confusion and uncertainty that has only heightened concerns and anxieties among those who oppose what they view as an all-out assault on women's reproductive rights.

Since the Supreme Court overturned the landmark Roe v. Wade ruling that had enshrined the constitutional right to abortion for half a century, around two dozen states have either banned or severely restricted access to abortion.

Regulatory chaos

The fresh battle over mifepristone has descended into a "judicial ping pong game," according to Carrie Flaxman, a senior director of public policy litigation with Planned Parenthood, a leading abortion provider and advocacy group.

The back and forth is impacting access to "safe and effective medication" and is "causing chaos and confusion," Flaxman said.

Ushma Upadhyay, a public health scientist at the University of California San Francisco, said it was "untenable" for providers and patients to be "living day to day under these ever-changing rules and interpretations."

Opposition to the legal attack on the abortion pill is being spearheaded by the Department of Justice, which argued that the initial judge's ruling was based on a "deeply misguided assessment" of the pill's safety.

The department urged the Supreme Court to preserve full access to the medication pending a full hearing at the appellate level, or take the case on itself on an "expedited" basis.

Mifepristone is one component of a two-drug regimen that can be used through the first 10 weeks of pregnancy. It has a long safety record, and the FDA estimates 5.6 million Americans have used it to terminate pregnancies since it was approved.

The Supreme Court, which has a 6-3 conservative majority, could decide to reinstate the restrictions put in place at the appellate level, the total ban, or some other configuration.

In a filing to the court on Tuesday, the anti-abortion coalition that first brought the case against the FDA repeated their controversial claims that the drug was unsafe. 

The coalition said the FDA and the distributor of mifepristone had "brazenly flouted the law and applicable regulations ... and continually placed politics above women's health."

Polls repeatedly show a clear majority of Americans support continued access to safe abortion, even as conservative groups push to limit the procedure -- or ban it outright.

Pregnancies are successfully terminated in more than 95 percent of cases where mifepristone is used, according to studies.

Serious complications -- excessive bleeding, fever, infection or allergic reaction -- which require a medical consultation, are rare.