While a majority of veterinarians may recommend spaying, greater disparity exists on the optimal age; approximately 16% of veterinarians recommend spaying before the first estrus cycle. A common justification for early spaying is the added benefit of protection against mammary neoplasia. In a review evaluating the association between mammary tumors and spaying or age at spaying, 13 reports addressed the association. Nine were judged to have a high risk for bias, and the remaining 4 to have moderate risk. An association between spaying and a reduced risk of mammary tumors was found in 1 study, but no evidence of an association was found in 2 other studies. Some protective effect of neutering on the risk for mammary tumors was noted in 1 study, but numbers were not presented. Because of limited evidence and risk for bias in published results, the evidence that neutering reduces risk for mammary neoplasia and age at spaying has an effect is not a sound basis for firm recommendations.

Commentary
Methodology used in this study may be unfamiliar, as it is more commonly used in human medicine. Cochrane Reviews are internationally recognized as high standards in evidence-based healthcare. Results of this study highlighted the need for quality research in veterinary medicine. Despite lack of evidence found to support early spaying as preventing mammary tumors, veterinarians may continue to recommend it to prevent estrus cycles, unwanted litters, and pyometra. Clinical experience may suggest that early spaying decreases the risk of mammary tumors, but without additional well-designed trials, scientific evidence to support this is lacking.—Ann Hohenhaus, DVM, DACVIM (Oncology)

Source
The effect of neutering on the risk of mammary tumours in dogs—A systematic review. Beauvais W, Cardwell JM, Brodbelt DC. J SMALL ANIM PRACT 53:314-322, 2012.

For More, see Early-Age Spay & Neuter by Dr. Phil Bushby