An Irish nanny accused of murdering a one-year-old girl in the US has had the charge against her dropped.

Aisling Brady McCarthy, 37, had been due to stand trial accused of killing one year-old Rehma Sabir in Massachusetts in 2013.

But prosecutors dropped the case after a state medical examiner reversed a finding that the baby's death was a murder caused by shaken baby syndrome, and instead said it was "undetermined".

The medical examiner made the change after finding that Rehma had past medical issues and may have had some type of undiagnosed disorder.

Marian Ryan, the District Attorney for Middlesex in Massachusetts, said: "Based on an assessment of the present state of the evidence, including the amended ruling from the medical examiner who performed the autopsy, the Commonwealth cannot meet its burden of proof."

Ms McCarthy is originally from Co Cavan but has been in the US illegally since 2002.

She spent almost two-and-a-half years in jail before being released on bail in May. It is not yet clear if she will be deported.

Ms McCarthy had been Rehma's nanny for around six months when the baby died in January 2013.

The medical examiner said the decision to change the cause and manner of death came after additional materials were reviewed.

These included expert witness reports, additional transcripts of police interviews, transcripts of testimony heard ahead of the planned trial, additional medical records and additional medical testing related to the girl's death.

The reports states: "These additional materials put forth several different and often conflicting opinions about the cause of Rehma's death.

"In particular the overall state of Rehma's health and her past medical issues raise the possibility that she had some type of disorder that was not able to be completely diagnosed prior to her death."

The report said Rehma had a history of bruising and that she might have been prone to easy bleeding with relatively minor trauma.

"Given these uncertainties, I am no longer convinced that the subdural hemorrhage in this case could only have been caused by abusive/inflicted head trauma, and I can no longer rule the manner of death as a homicide," the medical examiner added.

"I believe that enough evidence has been presented to raise the possibility that the bleeding could have been related to an accidental injury in a child with a bleeding risk or possibly could have even been a result of an undefined natural disease.

"As such I am amending the cause and manner of death to reflect this uncertainty."