ganesh there has been reams and reams of research published on this subject, but there are still conflicting reports so it is natural for vets to differ in their opinions depending on where they did their training. Urinary incontinence seems to be the main worry with early spaying, but the other health benefits far outweigh this, including mammary tumours and pyometra in later life, both of which can be fatal.
In one study 2.5% of early spayed bitches suffered some form of urinary incontinence, with a large proportion of that 2% being large breeds, whilst smaller breeds showed a much lower percentage. In an ideal world I would wait until after a first season, but in my case with three entire males it was not something that I was prepared to do, as an accidental mating and subsequent whelping or termination on a young bitch would have carried far more health implications.