A federal judge has temporarily blocked a Mississippi law that would ban most abortions once a fetal heartbeat is detected, at about six weeks of pregnancy.

“Here we go again,” US District Judge Carlton Reeves wrote in his order. “Mississippi has passed another law banning abortions prior to viability.”

His new order stops the law from taking effect on July 1.

Mr Reeves is the same judge who struck down a 2018 Mississippi law to ban abortion at 15 weeks.

Mississippi is one of several states that have pushed this year to enact bans on early abortions.

Alabama's Republican governor recently signed a law to ban most abortions, sparking protests in the state
Alabama’s Republican governor recently signed a law to ban most abortions, sparking protests in the state (AP Photo/Kim Chandler, File)

Opponents of abortion are emboldened by new conservative Supreme Court justices and are looking for ways to challenge the court’s 1973 ruling that legalised abortion nationwide.

Mr Reeves heard arguments on Tuesday from lawyers for the state’s only abortion clinic, who said the law would effectively eliminate all abortions in Mississippi because cardiac activity is often first detectable when many women might not know they are pregnant.

Lawyers with the state attorney general’s office said the law should be allowed to take effect because it was not a complete ban on abortion but was instead a limit on when the procedure could be done.

Alabama’s Republican governor recently signed a law to ban most abortions, while Georgia, Kentucky, Louisiana and Ohio have enacted or neared approval of measures barring abortion once there is a detectable fetal heartbeat.

Missouri politicians approved an eight-week ban. All of those laws are expected to face legal challenges, and the Kentucky one was temporarily blocked by a federal judge in March.

Mr Reeves ruled last year that Mississippi’s 15-week ban was unconstitutional because it would prohibit access to abortion before a fetus could survive outside the pregnant woman’s body.

People gather for a rally in support of reproductive rights in Dallas
People gather for a rally in support of reproductive rights in Dallas (Ryan Michalesko/The Dallas Morning News via AP)

Viability is generally considered to be about 23 or 24 weeks.

In an indication of which way he was leaning on the request to block the new law with the earlier ban, Mr Reeves asked lawyers on Tuesday: “Doesn’t it boil down to: Six is less than 15?”

Also during the hearing, Mr Reeves criticised Mississippi politicians for passing an earlier ban after he struck down the one at 15 weeks.

“It sure smacks of defiance to this court,” he said.

Mr Reeves will hear arguments later about the question of whether the six-week ban is constitutional. He wrote on Friday that the new law “prevents a woman’s free choice, which is central to personal dignity and autonomy. This injury outweighs any interest the State might have in banning abortions after the detection of a fetal heartbeat”.

The state is appealing Mr Reeves’ ruling on the 15-week ban, and Republican Governor Phil Bryant signed the new law in March.

The state’s only abortion clinic, Jackson Women’s Health Organisation, quickly sued the state.

Mr Bryant said in a statement on Friday that he was disappointed in Mr Reeves’ ruling.

“As governor, I’ve pledged to do all I can to protect life,” Mr Bryant said. “Time and time again the Legislature and I have done just that.”

The Mississippi law says physicians who perform abortions after a fetal heartbeat is detected could face revocation of their state medical licences.

It also says abortions could be allowed after a fetal heartbeat is found if a pregnancy endangers a woman’s life or one of her major bodily functions.

Senators rejected an amendment that would have allowed exceptions for pregnancies caused by rape or incest.