Lewis said she stopped assisting Gosnell in abortions in 2007 after a procedure in which she was the de facto anesthetist.
"I was the person who had given her too much and I was concerned whether she would come up from anesthesia," Lewis said.
She said she told Gosnell about her fears during the procedure and he "told me to monitor her."
Gosnell, 72, is on trial for seven counts of first-degree murder - seven infants allegedly born alive during abortions and killed by cutting their spinal cords. If the jury finds him guilty, Gosnell could be sentenced to death.
He is also charged with a count of third-degree murder in the death of a Virginia woman prosecutors allege was overdosed on Demerol by Gosnell's untrained staff.
Lewis was also asked by Cameron to review files of about 20 abortion patients, all over 24 weeks pregnant - the latest that abortions can be legally done in Pennsylvania.
Gosnell attorney Jack McMahon has argued that estimating gestational age in late-term abortions can have a two-week margin of error and that Gosnell did not knowingly violate the law.
McMahon also worked to undercut Lewis' descriptions of overmedication and unsanitary clinic conditions.
"Isn't it true that you referred family to Dr. Gosnell?" McMahon asked.
"Yes, family members came there before and after I was employed for abortions," Lewis said, adding that her father was a longtime patient in Gosnell's family practice.
Lewis said she had a "medical assistant" diploma from an eight-month program at the former Thompson Institute in West Philadelphia. The degree allowed her to draw blood samples, start intravenous lines, and inject medicine.
After she stopped assisting abortions, Lewis said, she worked the reception desk, registered patients, did some ultrasound exams, and distributed prescription slips to patients.
Lewis was not charged in the murder case involving Gosnell. According to earlier testimony, she began cooperating with authorities investigating Gosnell's medical practice even before the Feb. 18, 2010, raid that closed the Women's Medical Society and led to charges against Gosnell and nine workers.
Contact Joseph A. Slobodzian at 215-854-2985, firstname.lastname@example.org, or follow @joeslobo on Twitter.