Acetaminophen, most commonly sold as Tylenol, is practically the only medication that doctors say pregnant women can safely take for pain. But more than a dozen lawsuits are challenging that long-held belief, claiming that an increasing number of scientific studies show prenatal exposure to acetaminophen actually increased a baby’s risk of developing ADHD or autism spectrum disorder.

Mikal Watts of Watts Guerra, who has filed four lawsuits, said the litigation could be huge.