A 13-year-old girl and her newborn son were found safe in West Virginia on Saturday, a week after police say the teenager's stepfather kidnapped her from the Illinois home of her court-appointed guardian.

Police came upon Katherine Derleth, her baby and 39-year-old Christopher M Derleth in a remote campsite near the mining town of Kayford, West Virginia, located more than 500 miles from Edwardsville, Illinois, where the new mother and son were last seen on September 17.

Mr Derleth, who was charged last week in absentia with aggravated kidnapping and child abduction, was taken into custody and will remain in West Virginia pending his extradition to Illinois.

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Found: Katherine Derleth (left) and her three-week-old son, Christopher (right), were found safe at a remote campsite in West Virginia on Saturday, a week after their alleged kidnapping

Stepdad arrested: Katherine's stepdad, Christopher Derleth, 39, was taken into custody in West Virginia, where he will remain pending his extradition to Illinois

Katherine, who reportedly has a congenital heart defect that requires the use of a pacemaker, and her three-week-old son, also named Christopher, were taken to a hospital for an evaluation. Both were said to be in good condition.

Kanawha County Sheriff’s Sgt. Brian Humphreys said the elder Christopher Derleth used his credit card at a convenience store near the town of Cabin Creek and was seen on surveillance video on Friday, which enabled law enforcement officials to track him down at 1.30pm the following day,reported the St Louis Post-Dispatch.

The Madison County Sheriff's Office on Sunday issued an endangered missing person alert stating that Katherine Derleth and her son were last seen on September 17 in Edwardsville, Illinois.

On Wednesday, Katherine's mother, Jennifer Derleth, sat down with the station KDSK for an exclusive interview about her daughter's disappearance.

Mrs Derleth, who has a medical condition affecting her lungs that requires the use of an oxygen tank, recounted how three weeks ago, she was by Katherine's side in the hospital as the eighth-grader delivered her son via C-section after 36 hours of labor.

'I was there when he was born. She was nervous, scared,' the mom recalled.

Long way from home: This map shows the site of Katherine Derleth's disappearance in Edwardsville, Illinois, and the campsite where she was found in West Virginia, 500 miles away

Katherine's mother, Jennifer, pictured here with her husband, Christopher, and twin daughters, said last week that the 13-year-old Katherine wants to keep her son

She also said that when Katherine went missing from the home of her guardians Saturday, she left her cell phone behind but grabbed formula for baby Christopher.

Jennifer Derleth said the 13-year-old girl loves to cook and take care of people, and plans to keep the child.

A 'no contact order' had been previously issued against Derleth barring him from interacting with his stepdaughter

Citing legal considerations, the girl's mother declined to discuss why her husband may have abducted her daughter and grandson. She did say that the elder Christopher Derleth is an avid camper and asked the public to be on the lookout for his green 1997 Mercury Villager van.

The paternity of Christopher Ray Derleth's paternity is unknown at this time, but Greenville, Illinois, Police Chief Lou Lorton tells People Magazine that once investigators determine the identity of the baby's biological father, 'there will no doubt be charges of criminal sexual abuse filed against that individual.'

Lorton says his officers have launched a probe into the circumstances surrounding Katherine's pregnancy months earlier, and the focus of the investigation has been her stepfather and another man with whom the teenager had been sharing a home.

New details: Katherine has a congenital heart defect that requires the use of a pacemaker

The trio was believed to have been traveling in this green 1997 Mercury Villager minivan with Illinois plates

As a result, the state removed Katherine and her newborn son from her home and placed them with court-appointed guardians.

The foster family with whom Katherine and her son had been staying contacted the authorities at around 9.30am Sunday when they discovered that Katherine and baby Christopher were not in their room.