People are often uncertain about whether or not to use full stops in abbreviations. Here are some guidelines:
1. In both British and American English, if you are using initial (i.e. first) letters to represent words, you don’t normally need to put a full stop after them:
BBC（British Broadcasting Corporation）CNN（Cable News Network）UK（the United Kingdom）NATO（the North Atlantic Treaty Organization）IAEA（International Atomic Energy Agency）
2. In American English, however, it is common to use a full stop as an alternative style for certain abbreviations, in particular:
USA or U.S.A.US or U.S.
3. If an abbreviation consists of the first and last letters of a word, you don’t need to use a full stop at the end:
4. If the abbreviation consists only of the first part of a word, then you should put a full stop at the end:
Wed. （Wednesday）Dec. （December）