The heat or estrous cycle is the period of sexual receptivity in female dogs. The age and duration by which the dog starts getting heat depends upon the size and the breed of the dog. Normally, the heat cycle in dogs occur at an age from 6 - 12 months, and in smaller breeds it can be as early as 5 months, whereas in larger breeds the cycle may not begin until the dog is 14 months or sometimes even later. The cycle usually occurs twice a year, and the duration lasts for approximately 3 weeks. During this period, the female dog is receptive to mating with male dogs and has a high chance of getting pregnant. To have a better understanding of the signs of a dog in heat, let's break down the various stages of the average 3 weeks cycle.

Dogs in Heat Cycle
Like humans, the canine estrous cycle has different stages that vary in duration, and during which the dog undergoes different behavioral and physical changes. The estrous cycle in dogs has four stages - proestrus, estrus, diestrus and anestrus.

Proestrus - Symptoms
It is the first stage of the heat cycle which lasts from 7 - 10 days. Here are symptoms of dogs in heat for this stage:
  • The changes in the dogs may vary from quite mild to more severe. Some dogs may become more affectionate and clingy to their owner while others may seem a bit grumpy or fussy.
  • It's quite unusual for your pet dog to undergo appetite changes, as very rarely the dog may go off her food or become hungry to the point of raiding the trash bin for discarded leftovers. But whatever may be the change, keep in mind as it can be significant sign that the heat period has begun.
  • Another sign is the swelling of the vulva which can be noticed trailing up to the pelvic opening just below the anus. Along with the swelling, there will be bloody discharges from the vulva which is less during the initial few days but becomes a bit heavier till the mid-week.
  • The dog will try to guard the vulva, either by tucking the tail between the leg or sitting down whenever another the male dog approaches the immediate area.
Estrus - Symptoms
The onset of the estrus characterizes the fertile portion in the female dog's heat cycle, where the ovaries start to release eggs for fertilization. The period lasts from 5 - 7 days, and the symptoms include:
  • Pinkish-tan lightened discharge from the vulva is the first sign for the onset of estrus.
  • The initial swelling of the vulva subsides and it becomes soft enough for penetration.
  • Since the female dog is now ready for fertilization, she will start flagging her tail and behave flirtatiously to invite male dogs.
Diestrus - Symptoms
As diestrus starts, the fertile portion of the canine estrus cycle comes to an end and the female dog is less receptive towards the male dogs.
  • During this time, most of the swelling disappears but the vulva still remains slightly enlarged.
  • Whether bred or not, the female dog now lacks the conditions to mate and is no longer interested in mating.
  • The pinkish-tan vaginal discharge will turn red again but will gradually tapers off over the course of the final week.
Anestrus - Symptoms
This is the resting period that lasts for 5 - 6 months in which the female dog undergoes no hormonal changes and prepares herself for the next heat cycle.

During the heat cycle, moderate exercises, short walks or small periods of gentle play can keep the dog active, as neither excessive rest nor strenuous exercise is recommended for the dog's health. I hope the article will help you to understand the behavior of dogs in heat, and what necessary measures and care you need to take of your pets during those days.