The mother in this tragic case -- 20-year-old Jessica Ross -- says she was rushed to Southern Regional Medical Center in Clayton County, GA, back in July after her water broke ... after which, she went into a lengthy labor.

Ross says her baby boy, Treveon Isaiah Taylor Jr., became stuck 10 hours into her attempts to push him out -- and she has since claimed the doctor working on her tried to pull the baby out of her birth canal, allegedly using so much force his head detached from his body.

The mother claims the bones in her baby's skull, head and neck were broken by the doctor manipulating the infant's body way too hard ... which literally left the infant separated at the head and torso. When they did a C-section, the family claims the medical staff was able to pull the body out of the mother's stomach ... the head was still lodged in the birth canal.

The family has also alleged that the mother then had to deliver the baby's head vaginally -- just an absolutely awful thing as described in a lawsuit they subsequently filed. What's more, the family alleges the hospital tried to cover up the baby's decapitation ... allegedly wrapping their dead child in a blanket so as not to show his severed head.

On Tuesday, the Clayton County Medical Examiner's Office ruled the death a homicide and said the baby died from "fracture-dislocation" in his spinal cord and upper cervical spine.

The family is now going after the hospital, doctors and nurses involved in the delivery ... suing for wrongful death and medical negligence.

Meanwhile, the Clayton County Police Department says it's launching a criminal investigation.

In a statement, the hospital says the baby died "in utero prior to the delivery and decapitation."