The verdict is the first of about 600 similar Paxil lawsuits filed around the country.
The jury found the company guilty of negligence but not outrageous conduct, and rejected punitive damages.
Plaintiff Michelle David of suburban Philadelphia has a son born four years ago with several heart defects. The child spent months in the hospital and has had several surgeries. The family's lawyer says he will need at least one more.
GlaxoSmithKline says it plans to appeal the verdict.
The Food and Drug Administration issued warnings in 2005 that Paxil may be associated with birth defects.