Olympic gymnastics gold medalist McKayla Maroney says the former US team doctor facing child porn and sexual assault charges "deserves to spend the rest of his life in prison".

Larry Nassar is scheduled to be sentenced in federal court in Michigan on Thursday after pleading guilty to child pornography charges.

He has also pleaded guilty to 10 counts of sexual assault in two other cases in Michigan, admitting that he abused young athletes under the guise of offering medical treatment.

Maroney and Olympians Aly Raisman and Gabby Douglas have said they, too, were assaulted by Nassar, who worked with USA Gymnastics as a trainer and medical coordinator.

"He abused my trust, abused my body and left scars on my psyche that may never heal," Maroney wrote in a victim-impact letter to the federal court where Nassar is to be sentenced on Thursday. The letter was obtained by ESPN's Outside the Lines program.

"Larry Nassar deserves to spend the rest of his life in prison," Maroney wrote.

Maroney won team gold and an individual vault silver medal at the 2012 London Olympics. A three-time World Champion, she first went public with her allegations against Nassar amid the "#MeToo" movement.

She said Nassar began abusing her when she was just 13, and the abuse continued throughout her gymnastics career.

"It happened in London before my team and I won the gold medal, and it happened before I won my silver medal," Maroney wrote.

Maroney's mother, Erin Maroney, says in a letter to the court that abuse changed her daughter.

"She was transformed from a bubbly, positive, loving world class athlete into a young adult who was deeply depressed, at times suicidal," Erin Maroney wrote.

According to court documents, prosecutors have asked for a 60-year-sentence for Nassar on the child pornography charges.