At the sentencing for Katie Stockton, 32, investigators described finding the infant, named Baby Crystal, wrapped in a plastic bag with bloody clothes.
Michael Pearson of the Winnebago County Sheriff's Department said he found a "significant" amount of blood and when he looked closer in the bag he saw the backside of "a small, newborn baby that was frozen," WREX-TV, Rockford, reported.
Stockton, who has been jailed since her arrest in 2009, pleaded guilty to first-degree murder in February.
The Rockford Record reported an autopsy showed the infant, found on land owned by the family, was born alive and died of suffocation.
Stockton, who had a drug habit, hid her pregnancy from her family, friends and co-workers and gave birth on Dec. 17, 2004.
The Rockford Register Star said police later found the skeletal remains of two other baby girls inside Stockton's car. Authorities could not determine whether they had been born alive and she has not been charged in those deaths.