In the IELTS writing exam, it is generally recommended to avoid using abbreviations unless they are very common and widely understood. Abbreviations can sometimes confuse the reader, especially if they are specific to a certain region or field. It's important to ensure that your writing is clear and easily understandable to the examiner, who might come from a different cultural or linguistic background.
However, there are a few exceptions where commonly used and well-known abbreviations can be used. For example, abbreviations like "USA" (United States of America), "UK" (United Kingdom), "UN" (United Nations), and similar internationally recognized abbreviations are generally acceptable to use.
It's always safer to write out the full form of words and phrases to avoid any potential confusion. If you are unsure about a specific abbreviation, it's best to write out the full form to ensure clarity and demonstrate your language proficiency effectively.