Defence lawyers say Stephanie Greene, a former nurse , suffered from chronic pain before her pregnancy. (Spartanburg County Prosecutor's office/AP)

A judge sentenced a woman to 20 years in prison Friday for killing her six-week-old daughter with what prosecutors say was an overdose of morphine delivered through her breast milk.

Stephanie Greene, 39, said nothing as the minimum sentence was handed down at a court in Spartanburg, South Carolina. A jury found the former nurse guilty of homicide by child abuse the day before and she could have faced up to life behind bars.

Her lawyer said she will appeal and it's likely the case will be tied up for years to come. Both the prosecutor and Greene's lawyer agree no mother has ever been prosecuted in the United States for killing her child through a substance transmitted in breast milk.

Greene's daughter Alexis was born healthy, but was found dead in her parents' bed just 46 days after she was born in November 2010.

An autopsy found a level of morphine in the baby's body that a pathologist testified could have been lethal for an adult. With no needle marks on the child's body, authorities decided the drugs must have gotten into the infant through her mother's milk, prosecutor Barry Barnette said.

A review of her medical records showed Greene carefully hid her pregnancy from her primary doctor. After a home pregnancy test showed she was pregnant, she told her primary doctor she needed to go to a gynecologist for a birth control. She then got prenatal care from that doctor while not telling her all the painkillers she was taking. She also skipped appointments with her primary physician when it was obvious she was pregnant and sent her husband to pick up her painkiller prescriptions, Barnette said.

"She was a nurse. She knew how to work the system," Barnette said. "She caused the loss of that child."

Lawyer disputes evidence

Greene spent more than 10 years racked with chronic pain after a car wreck before her unexpected pregnancy with her husband in 2010, attorney Rauch Wise said.

Wise argued that prosecutors didn't prove how the baby got the morphine and there is little scientific evidence that enough morphine can gather in breast milk to kill an infant.

Greene already suffered an immeasurable loss with the death of her child and shouldn't have to face prison time, Wise said.

"Society wants to portray people who need painkillers as drug addicts and horrible people, but Greene and others often are just trying to get through each day without debilitating pain," her lawyer said.

"She needed those meds to get up in the morning and function," Wise said. "She was on total disability because of her pain, her fibromyalgia and all the other things wrong with her."

Greene will have to serve 16 years in prison before she is eligible for parole.