For decades, the standard in the veterinary profession was to spay female dogs and cats at the age of six months, and neuter males at nine months. This standard has contributed significantly to the tragedy of pet overpopulation, since most cats and many dogs have reached physiological sexual maturity by that time. Fortunately, the standard is changing. Female dogs can come in season (and get pregnant) prior to age six months. A dog's estrus cycle can be messy, leaving spots of blood on carpets and furniture, and a dog in heat can be a real nuisance. Every unsterilized male dog for miles around will make supercanine efforts to reach her.