The word "estrous" refers to a specific stage in the reproductive cycle of female mammals, typically associated with being in heat or the period when they are most likely to mate and reproduce. Full definition
However, spaying your female dog before her first estrous cycle greatly reduces the risk of developing mammary tumors. (dogcare.dailypuppy.com)
The earlier you spay the better — performing the operation before her first estrous cycle lowers her risk of mammary cancer later in life and can stop her from developing bad habits like urine - marking. (dogcare.dailypuppy.com)
Other causes for urine marking involve exposure to social triggers, such as encountering a female dog in estrous (in heat). (pets.webmd.com)