Is it safe to take mucinex (guaifenesin) dm while pregnant?

Pregnancy Category C: Pregnancy category c means animal reproduction studies have shown an adverse effect on the fetus and there are no adequate and well-controlled studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks. You should discuss it with your doctor whether taking it is a good idea.

Category C: Category a is safe. Category b likely safe, no evidence in humans. Category c is no evidence of harm in humans for sure but possible in animals. So there could be some risk.