With bitches the benefits if spaying in IMO massively outweigh the disadvantages risks. 1. Urinary incontinence. A large retrospective study in 2000 looked at thousands of bitches when they were spayed and urinary incontinence. It found that the bitches who are going to get urinary incontinence get it regardless of when we spay them. 2. No study has conclusively shown an effect on joints by early spaying, but the current feeling is that giant breed (dogs whose healthy adult weight will be over 45kg) should have a season. 3. There is a massive reduction in the risk of mammary tumours if the bitch is spayed before the third season after the third season you loose the protective effects of spaying. Then there is your personal preference as to whether you want to deal with a season or not.
We were told both by the dogs home and the vet to have her spayed after 1 season. Before that you risk her being in heat or about to be and that's not good. Let her have 1 season and then book her in 3 months from the end of the heat then she's definitely not about to come into heat again. That's what we were told anyway. My dog is just ending her first heat. Not sure how to be sure it's safe again though