Apple cider vinegar in food amounts (salad dressings, marinades, cooking) is safe during pregnancy. However, drinking straight ACV or large amounts can damage tooth enamel and irritate the esophagus. Unpasteurized ACV (with "the mother") has not been well studied in pregnancy.
A "Use Caution" rating means this food is generally fine during pregnancy but comes with a specific factor worth managing — a daily limit, a preparation requirement, or a moderation guideline — rather than a reason to avoid it outright. These ratings reflect general guidance from the FDA, ACOG, CDC, and USDA. The goal of the "caution" label is to flag what to pay attention to, not to suggest the food is unsafe when that factor is properly managed.