
It used to be postulated that 50% of bitches developed mammary tumours and of those around 40% were malignant.
More recently the study that concluded this was found to be erroneous as it was a small sample f related individuals it was based on.
In my own experience of owning 9 bitches that are/were 3 or over in my breed, I have only had one with a small mammary tumour, that was malignant and removed. She died of old age at 13 1/2.
All mien have been spayed sometime between 5 and 7 (the one with the lump was 5, as I erroneously thought it would be helpful).
By leaving a bitch past her third season the RISK of mammary tumours is said to increase by 25%, but then again not allowing an organism to fully mature before interfering with it may cause more health and mental issues. it is easy enough to check a bitches mammaries and have an lump investigated when small.