A woman initially hired to clean instruments at a Philadelphia abortion clinic has pleaded guilty to two counts of third-degree murder in the deaths of a newborn baby and a woman who died after an anesthesia overdose.

As part of her plea agreement with prosecutors, Lynda Williams also agreed on Wednesday to testify against the operator of the clinic, Dr. Kermit Gosnell.

Williams was one of 10 people charged in a shocking grand jury report that alleged viable, live-born babies were routinely killed at Gosnell's clinic by having their spinal cords severed with scissors.

At the end of the hearing, a prosecutor told the teary-eyed Williams she "did the right thing" by pleading guilty, The Philadelphia Inquirer reported.

The grand jury report described filthy, inhumane conditions at the clinic, which served many poor and immigrant women.

According to the grand jury report, Gosnell hired Williams, 43, in 2008 to clean instruments at his Women's Medical Center in West Philadelphia. But her duties soon increased to include performing ultrasounds and administering anesthesia. Authorities said it was Williams who administered a lethal mix of drugs that killed Karnamaya Mongar in November 2009.

Mongar was 19 weeks pregnant when she traveled from Virginia to Philadelphia for an abortion. The Bhutanese refugee died after Williams, of Wilmington, Del., gave her a mixture of Demerol, promethazine and diazepam.

Gosnell, the only licensed doctor at the clinic, typically did not arrive until late at night to perform abortions, leaving untrained, unlicensed staff to give anesthesia, monitor patients and perform other skilled care, prosecutors allege. The grand jury said state regulators did not inspect the clinic for years.

Four other suspects have already pleaded guilty in the case, including an unlicensed doctor who told the grand jury he snipped the spinal cords of more than 100 babies after seeing them breathe, move or show other signs of life.

Gosnell, 70, and his wife, Pearl, are awaiting trial. Gosnell is charged with first-degree murder and could face the death penalty if convicted.