Our breeder strongly suggested we not spay our girl until a year. We got a boy the second time & this time it is written in the contract, & voids health guarantees if we spy/neuter either of them before one.
She sent me lots of research & after reading I agreed. The increase incidence of mammary cancer in a dog who had only one heat cycle is minimal, like less than 1% increase. However their is a significant increase in development of excessive tibial plateau angle (TPA) in large-breed dogs with cranial cruciate ligament disease (CCLD) who were spayed by 6 months.. There is also an increase of spay incontinence in female dogs, and an increase in cancer in males if done early.