(Reuters) -A U.S. judge said several companies including Walmart ( WMT ), Beech-Nut and Gerber must face a nationwide lawsuit claiming that toxic heavy metals contaminated their baby food, causing brain and neurodevelopmental damage to children who ate it.

In a decision on Wednesday, U.S. District Judge Jacqueline Scott Corley said parents can try to prove that defective manufacturing, negligence and failure to warn about more than 600 baby food products caused their children to suffer autism spectrum disorder and attention deficit hyperactivity disorder.

Parents said some defendants failed to adhere to internal limits about how much arsenic, cadmium, lead and mercury in baby food was safe, while others never addressed the issue.

Corley said this made it plausible to claim that some baby food was unsafe, if safety criteria were not followed. The San Francisco-based judge also said no "ironclad rule" required the parents to allege that toxicity crossed a particular threshold.

Beech-Nut is owned by Nestle, Gerber is owned by Switzerland's Hero Group, and Walmart ( WMT ) sold its baby food under its own name.

Other brands in the case include Hain Celestial's ( HAIN ) Earth's Best Organics, Danone's Happy Baby and Happy Tot, Sun-Maid Growers of California's Plum Organics and Neptune Wellness Solutions' Sprout Organic.

Lawyers for the defendant companies did not immediately respond to requests for comment on Thursday.

The companies have said their baby food is safe. They also argued that heavy metals are naturally present in the environment, and parents "cannot simply allege that detectable levels of heavy metals make baby food defective."

R. Brent Wisner, a lawyer for the plaintiffs, said he was pleased with the decision.

"Selling baby food with lead and arsenic is simply not OK, and with the court's ruling, we are one step closer to holding these companies accountable for their decades of malfeasance," Wisner said in an email.

Parents sued after a 2021 report by a U.S. House of Representatives subcommittee on economic and consumer policy said "dangerous" levels of heavy metals in some baby food could cause neurological damage.

Corley dismissed Campbell's, which sold Plum Organics to Sun-Maid in 2021, as a defendant.

Amazon.com ( AMZN ) and its Whole Foods unit have also been sued for selling Hain and Danone baby food.

The case is In re: Baby Food Products Liability Litigation, U.S. District Court, Northern District of California, No. 24-md-03101.