An American woman who murdered a pregnant dog breeder in order to steal her baby was put to death by lethal injection on Wednesday, becoming the first female to be executed by United States federal authorities in nearly seven decades.

The US Justice Department said Lisa Montgomery, 52, was pronounced dead at 1:31am Eastern Time (0631 GMT) at a penitentiary in Terre Haute, Indiana.

It said the execution was “in accordance with the capital sentence unanimously recommended by a federal jury and imposed by the US District Court for the Western District of Missouri”.

The US Supreme Court cleared the way for Montgomery's execution just hours earlier — despite doubts about her mental state — after the government of President Donald Trump had pushed for the application of the death penalty.

Challenges were fought across multiple federal courts on whether to allow execution of Montgomery, 52, who had initially been scheduled to be killed by lethal injections of pentobarbital, a powerful barbiturate on Tuesday in the Justice Department’s execution chamber at its prison in Terre Haute, Indiana.

Montgomery's defenders did not deny the seriousness of her crime: in 2004, she killed a pregnant 23-year-old in order to steal her baby.

But her lawyer Kelley Henry, in a statement, called the decision — the first for a female inmate since 1953 — a “vicious, unlawful, and unnecessary exercise of authoritarian power".

“No one can credibly dispute Ms Montgomery’s longstanding debilitating mental disease — diagnosed and treated for the first time by the Bureau of Prisons’ own doctors,” Henry said in a statement.

Montgomery was convicted in 2007 in Missouri for kidnapping and strangling Bobbie Jo Stinnett, then eight months pregnant. Montgomery cut Stinnett’s fetus from the womb. The child survived.

Some of Stinnett’s relatives have traveled to witness Montgomery’s execution, the Justice Department said.

Montgomery's defenders believe that she suffered from severe mental health issues stemming from abuse she suffered as a child. She did not understand the meaning of her sentence, they said, a prerequisite for execution.

On Monday evening, a federal judge offered the defence a brief lifeline, ordering a stay of execution to allow time to assess Montgomery's mental state.

“The record before the Court contains ample evidence that Ms Montgomery's current mental state is so divorced from reality that she cannot rationally understand the government's rationale for her execution,” the ruling stated.

But an appeals court overturned that decision on Tuesday, leaving it up to the US Supreme Court to decide, which said the execution could go ahead.

Clemency plea ignored

Trump, like many of his conservative constituents, is a strong supporter of the death penalty and ignored a plea for clemency from Montgomery's supporters.

Despite the decline of capital punishment in the US and around the world, Trump's administration resumed federal executions in July after a 17-year hiatus and has been carrying them out at an unprecedented rate ever since.

Since the summer, 10 Americans have died by lethal injection in Terre Haute. In addition to Montgomery, two men are scheduled for federal execution this week. Their executions were stayed on Tuesday due to them having contracted Covid-19.

Democratic Senator Dick Durbin on Monday announced the introduction of legislation to end federal executions. It could be passed once president-elect Joe Biden takes office next week and Democrats regain control of the Senate.

Former guards of the penitentiary in Terre Haute have written to the Justice Department to request that the executions be postponed until the penitentiary staff are vaccinated against Covid-19.

Between the executioners, guards, witnesses, and lawyers, an execution assembles dozens of people in a closed environment, which is conducive to the spread of the virus.

US states, including the deeply conservative Texas, have suspended executions for months due to the pandemic — unlike the federal government, which has pushed to carry out many before Trump leaves power.