Pus bubble or blister under the tongue

My six month old has a clear fluid filled sac under his tongue. What could this be?

[Arti Sharma, M.D.]

Hello,
I can confirm the exact cause only after a clinical examination. Your child probably has a ranula, a type of cyst that forms on the floor of the mouth. It is a cyst which is a fluid filled sac of saliva or thin mucous material. These need to be removed by an oral surgeon as they do not just go away. Other possibilities are oral cysts, mucocoele.Get it examined by a doctor and confirm the diagnosis. Treatment will be specific to the cause.
Best of luck and regards!