Collins said he listed the manner of death as accidental in an Aug. 13, 2010, first draft of Mongar's death certificate. He reiterated the conclusion in a second draft dated Aug. 31, 2010, that he and Chief Medical Examiner Sam P. Gulino initialed.
But when Mongar's final death certificate was filed on Dec. 17, 2010, the manner of death was listed as homicide.
Why the change was made is important to Gosnell's case because Mongar's death is the basis of a third-degree murder charge against him.
Collins said he only began to revise his conclusion in September 2010, after a county investigating grand jury was impaneled and the District Attorney's Office began providing him with additional evidence.
He said he got statements police took right after Mongar's death from Mongar's daughter and three workers present the night she died at the Women's Medical Society clinic at 3801 Lancaster Ave.
He also toured the abortion clinic in October 2010, he said.
"You're telling us that during that nine-month period, you didn't see any of these statements?" McMahon asked.
"We didn't know they were out there," Collins replied. "This is not TV. We can't go out and interview people. We are bound by the limitations of the information provided to us."
When he first ruled the death accidental, Collins added, he assumed Gosnell's clinic was a legitimate abortion provider: "I assumed it was a medical institution - and I'm using that word loosely - with a trained staff and working equipment doing proper procedures."
The byplay between Collins and McMahon got steadily more heated until Common Pleas Court Judge Jeffrey P. Minehart had to warn them.
McMahon appeared to be seething when at one point, Collins stood, ripped one of the lawyer's charts from an easel, and began creating his own exhibit.
Minehart later told the jury to focus on the evidence, not the histrionics of lawyers, witnesses, and "sometimes a judge."
Collins' testimony ended eight days of a trial estimated to last six to eight weeks.
Minehart is not holding court on Fridays, so the trial resumes Monday, when Assistant District Attorneys Edward Cameron and Joanne Pescatore will continue the prosecution's case.
The prosecution alleges Mongar died because Gosnell's untrained workers gave the diminutive woman multiple doses of Demerol and other drugs to anesthetize her that instead stopped her heart.
McMahon has argued that Mongar hid respiratory problems that made her more vulnerable to the effects of Demerol.
In addition to the charge involving Mongar's death, Gosnell faces seven counts of first-degree murder in the deaths of newborns whose spines he allegedly snipped with scissors after late-term abortions. He faces the death penalty if the jury finds him guilty.
Also on trial is Eileen O'Neill, 52, of Phoenixville, an unlicensed medical school graduate who worked as a doctor in Gosnell's clinic. She is not charged with performing abortions.
Contact Joseph A. Slobodzian at 215-854-2985, firstname.lastname@example.org, or follow @joeslobo on Twitter.