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.