What does a fishy smell to a babies breath mean?

See below. There is a rare genetic disorder called "trimethylaminuria" where an enzyme deficiency is present causes the lack of breakdown of a particular metabolite can cause the accumulation of trimethylamine in sweat, saliva and urine causing a fishy odor. The malodor can be managed by reducing the ingestion of precursors of the compound. See your doc for an eval and referral to genetics.