Drugmaker Pfizer Inc. has been aggressively attacking the experts the plaintiffs want to use to show a causal relationship between Zoloft and birth defects. Now the battle over those experts has moved into the courtroom, with hearings this week in Philadelphia on the admissibility of their opinions.

Anick Bérard, a perinatal epidemiologist who studies babies’ exposure to drugs before they are born, has received the most extensive attention from Pfizer’s lawyers.

Pfizer argues that Bérard opined in earlier litigation that the antidepressant Paxil increased the risk of cardiac defects but that other antidepressants, including Zoloft, did not.

Pfizer also argues that there is no class effect for selective serotonin re-uptake inhibitors, so data about other antidepressants should not be included in Bérard’s analysis.

Lead plainitffs lawyers Mark Robinson Jr. and Kevin Calcagnie of Robinson Calcagnie Robinson Shapiro Davis, and Dianne Nast of NastLaw LLC, argued in a document filed with U.S. District Judge Cynthia M. Rufe said that internal Pfizer documented support Bérard’s expert opinion. Those documents “show that Pfizer’s epidemiologists were utilizing the same methodology for which Pfizer criticizes Dr. Bérard—considering SSRIs as a class.”

Pfizer insisted there are other problems with Bérard’s proposed testimony.

“Plaintiffs ignore altogether multiple other methodological infirmities in Dr. Bérard’s report,” Pfizer counsel Sheila Birnbaum, Mark Cheffo, Bert Wolff and Jonathan Tam of Quinn Emanuel Urquhart & Sullivan in New York said. “For example, plaintiffs do not address the many statements that Dr. Bérard made to her peers in the scientific community that are flatly inconsistent with the for-litigation opinions she proffers here—including her peer-reviewed statements that Zoloft should be used as a first-line therapy during pregnancy.”

The plaintiffs said that the complaints about Bérard go to the weight of her testimony, not its admissibility.

Bérard “has applied an accepted, factually thorough and scientifically valid methodology, utilizing the totality of the evidence, to arrive at her conclusions,” the plaintiffs said.

Testimony by Thomas Sadler, an embryologist and development biologist; Robert Cabrera, who studies abnormalities in physiological development; and Michael Levin, director of the Tufts University Center for Regenerative and Developmental Biology, also will be taken up during the hearings this week.

Those three proposed experts argue that Zoloft alters the concentration of serotonin in the womb and thus affects the development of babies.