I have always seen the recommendation that we should first write unit tests and then start writing code. But I feel that going the other way is much more comfortable (for me) - write code and then the unit tests, because I feel we have much more clarity after we have written the actual code. If I write the code and then the tests, I may have to change my code a little bit to make it testable, even if I concentrate much on creating a testable design. On the other hand, if I write the tests and then the code, the tests will change pretty frequently as and when the code shapes up.
As I see a lot of recommendations to start writing tests and then move on to coding, what are the disadvantages if I do it the other way - write code and then the unit tests?