An Iowa couple was charged with murder after their four-month-old son was found dead and covered in maggots in a baby swing.

Cheyanne Renae Harris, 20, and Zachary Paul Koehn, 28, were arrested on charges of first-degree murder and child endangerment, the Des Moines Register reported.

Authorities found the couple's son, Sterling Daniel Koehn, dead on 30 August after Koehn called 911 for an ambulance, according to a criminal complaint. Chickasaw County sheriff's deputies and medics found the decomposing boy in a powered swing in a bedroom separate from the room the couple shared with their older child.

An autopsy of baby's body found "maggots in various stages of development in his clothing and on his skin". The autopsy also found that Sterling was underweight at less than seven pounds, putting him below the 5th percentile in size and weight, BuzzFeed News reported.

Court records reveal that forensic entomologists, who studied the maggots, determined that the boy had not been removed from the baby swing, had a diaper change or bath in more than a week.

Koehn reportedly told deputies who first arrived in the couple's home that Harris had fed Sterling that morning "and he was fine". Koehn said Harris checked on the infant hours later and he had died.

However, Koehn's story was "inconsistent" with the autopsy findings and investigation, court records show. The couple later amended their story and claimed they had last checked on Sterling the previous day.

The death was ruled a homicide by the Iowa State Medical Examiner and the cause of death was failure to provide critical care.

"The facts of this case go far beyond neglect and show circumstances manifesting an extreme indifference to human life," Chickasaw County Sheriff's Deputy Reed Palo said in the criminal complaints.

The couple was jailed in Chickasaw County. A preliminary hearing is scheduled for 2 November.