Gosnell, 72, is charged with third-degree murder in Mongar's death, which prosecutors say was caused when untrained workers gave the diminutive woman multiple doses of Demerol and other drugs to anesthetize her.
McMahon has argued that Mongar hid respiratory problems that made her more vulnerable to Demerol.
McMahon challenged Rohrig's ability to estimate how much Demerol Mongar received, and cited three studies he said concluded there is no reliable way to estimate drug levels in a living person based on blood samples taken after death.
After a hearing outside the jury's presence, Common Pleas Court Judge Jeffrey P. Minehart ruled that Rohrig's expertise and testimony met the legal standard.
Rohrig, a forensic toxicologist and director of the Sedgwick County, Kan., Regional Forensic Science Center, testified that records from Gosnell's clinic showed Mongar had been administered 150 milligrams of Demerol in the 24 hours before the abortion.
Rohrig told Assistant District Attorney Joanne Pescatore that clinic records were inconsistent with the 710 micrograms of Demerol found in a blood sample taken from Mongar's heart during autopsy.
Rohrig conceded there was no way he could accurately estimate how much Demerol Mongar had before she died. But he said that there was no way 710 micrograms could remain in her blood shortly after death if she had only been given 150 milligrams.
"It was more than 150 and probably a lot more than 150," Rohrig testified.
Under questioning by McMahon, Rohrig maintained that the studies McMahon cited were "cautionaries" warning forensic experts of the pitfalls of estimating drug levels in a live person from blood drawn after death.
"Yes, you're supposed to be careful so you don't go down the wrong path," Rohrig said, but added that his opinion has been admitted in testimony in more than 200 trials.
In addition to the charge involving Mongar's death, Gosnell faces seven counts of first-degree murder in the deaths of newborns whose spines were 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 @joeslobo on Twitter.