For intact female dogs, going into heat is an inevitable fact of life. When female canines go into heat, they enter a phase of being receptive to males for breeding. In dogs, heat generally occurs two times a year. It's also commonly known as estrus.
Being in heat or estrus is something of a hormonal roller coaster for female dogs. When heat begins, female dogs' estrogen counts rise, only to drastically fall. Estrogen is a female sex hormone. The rise and fall lead to the release of eggs via the ovaries. Young puppies are usually old enough to go into heat by their first birthday, though factors including breed mean some dogs are sexually mature by 6 months old and others not until 2 years old. Bigger dog breeds tend to reach reproductive maturity at slower paces than their tinier counterparts.
With the estrogen fluctuation going on, female dogs in heat usually exhibit strong hormonal behaviors. Dogs in estrus often have difficulty concentrating on things. They often seem extremely vigilant, fidgety, unpredictable and anxious. Some also behave in unusually loving, lively and animated ways, requesting more acknowledgement from their human owners. Some female dogs try to invite mating sessions with male dogs. They do this through body language -- stiff back legs and elevated behinds. These types of behavioral irregularities are not random. They're direct results of the females' changing levels of hormones.
When dogs are in heat, their urine consists of hormones and pheromones alike. These things notify nearby male dogs of their mating availability. If a dog is in heat, she might draw the attention of dozens of intact local male dogs, sometimes even from several miles away. Males often react to female dogs in heat by trying to claim the turf around them, generally by marking with urine. Male dogs, in contrast to females, don't ever go into heat. Healthy, intact mature males are willing and able to mate at any point.
Consensus among veterinarians is that you should spay your female dogs unless you intend to breed them. Spayed dogs no longer go into heat and so do not experience estrus-related behavioral fluctuations. They're also incapable of getting pregnant, which is beneficial for controlling populations. Risk of some cancers and tumors is virtually eliminated. Talk to your veterinarian about a suitable time period for spaying your pet. It isn't uncommon for puppies to undergo the procedure at merely 2 months old. Remember, however, that all dogs have varying health backgrounds and needs. Your vet can decide what's safest and most appropriate for your individual pet.