Florida's Supreme Court is to decide whether to execute a mentally impaired prisoner, who has spent over 30 years on death row for having killed a pregnant woman.

Freddie Lee Hall was declared mentally retarded when he was convicted in 1991.

However, in 2002's Atkins v. Virginia case, the Supreme Court ruled that executing people with mental retardation is unconstitutional. Hall's sentence has since been put on hold.

The Supreme Court is expected to revisit its 2002 decision, and to specify what mental retardation is, as a definition of the condition is still missing in the ruling.

According to Florida law, whoever scores an IQ of 70 and below is to be considered mentally disabled. According to experts, there is a marginal error of five points to be considered; someone who scores 75 can apparently still be considered mentally disabled.

Hall, whose IQ falls somewhere between 60 and 80, should be executed as he repeatedly scores more than 70.

"If the bar against executing the mentally retarded is to mean anything, Freddie Lee Hall cannot be executed," said Judge James Perry. Hall "is a poster child for mental retardation claims," said Perry.

"The Florida Supreme Court has redefined mental retardation so that it means something different — and narrower — than this court's decision contemplated," Perry said.

"The predictable consequence of Florida's rule is that persons with mental retardation will be executed. Without this court's intervention, that will happen here," Perry continued.

Attorney general Pamela Jo Bondi noted: "Florida's standard resulted in the correct outcome in this case.

"Hall's conduct in his crimes and extensive evidence from the record of his appeals refutes his claims of intellectual and adaptive deficits."

From 1984 to 2001, 44 people with mental retardation were executed, USA Today said.

Since the Supreme Court's ruling in 2002, several hundred claims of mental retardation have been filed by prisoners on death row; 28% of the sentences have been reduced as a result of the ruling.

Three people were executed in Florida in 2012, according to the Death Penalty Information Centre.

Capital punishment is legal in 32 states in the US. Out of the 3,170 prisoners on death row, 43 executions were carried out in 2012. Fifteen of these sentences were carried out in Texas.

Three quarters of all executions occurred in only four states: Texas, Arizona, Mississippi and Oklahoma.