Among the species that constitute the animal kingdom, only female primates ranging from the lemur to the human to the gorilla have periods in the way we generally understand the term. All mammals go through a period of what is called estrus, which is the equivalent of the human female period. A female dog will have her first estrus cycle when they reach puberty. It is the stage of the reproductive cycle where the dog is capable of becoming pregnant when there is a male dog to engage in reproduction. This period of estrus is also called “being in heat” or “in the season.”
When a female comes into “heat” or “season,” her body is preparing for breeding and the possibility of producing a litter. According to the University of Georgia College of Veterinary Medicine, the cycle is broken into stages.
- Proestrus – This stage typically lasts about nine days, and it’s marked by an increase in a female’s estrogen level. She won’t be receptive to a male’s advances just yet, but she will show many of the signs of heat listed below.
- Estrus – During this stage which also lasts about nine days, estrogen levels drop while progesterone levels rise. The female will begin ovulating during this stage, which means a series of eggs will be released from her ovaries and become available for fertilization. She will now begin to be receptive to a male’s attempts to breed her.
- Diestrus – This stage lasts about two months. Progesterone levels are still elevated, but the female will no longer be receptive to a male’s attempts to breed with her.
- Anestrus – This is the resting stage that lasts until the female comes into heat again.
Menstrual cycle vs. estrous cycle
No female dogs don’t have periods in the same way that human females do. While mammals of all sorts including dogs and humans share the same basic reproductive organs, the ways those organs function is not similar. Human women go through a menstrual cycle, a process of preparation for egg fertilization that lasts an average of 28 days. Female dogs, on the other hand, go through an estrous cycle, similar in purpose, but different in execution, which lasts an average of 180 days.
In humans, the uterus builds up nutrients for the anticipated growth of a fetus. When an egg goes unfertilized, that material is secreted from the body. In dogs, when an egg is unfertilized, that nutrient-rich material is absorbed by the body over an extended period of time. The bloody discharge that emerges from female dogs originates in the vagina, not the uterus, and serves a different function.
The estrous cycle in dogs
Dogs do not menstruate. The estrous cycle in dogs consists of four major phases: proestrus, estrus, diestrus, and anestrus. A dog’s vagina discharges blood and other fluids during the first two of these phases, most heavily during proestrus. There are two phases proestrus and estrus.
During proestrus, which can last from three to 17 days, a female dog’s body produces large amounts of estrogen. In dogs, this is accompanied by the start of bloody vaginal discharge, which is dark red to begin with, and caused in part by excessive hormone and pheromone production. Dogs also urinate much more frequently during proestrus.
Estrus is the shortest part of the estrous cycle, lasting from four to seven days. This is typically when a dog is primed for mating and fertilization. During estrus, bleeding tends to continue, though it may slow and take on a lighter tint. Discharge in estrus can range from a lighter shade of red to pink to straw-colored. In this phase, a dog may sleep more, be less inclined to play, and begin building a nest in anticipation of pregnancy.
Signs a Female Is in Heat
Common signs that you can expect to see when a female is in season include:
- Mood change – Some females show a change in mood shortly before their season commences, and they may even act a bit touchy. Think of it as the doggie equivalent of PMS.
- Sudden interest from males – Males are great early warning detectors and can smell the change in a female’s hormones before she fully comes into heat.
- Tail flagging – When a female is ready to be bred, she’ll usually stand quite still while the male investigates her vulva. She’ll hold her own tail up and wag it side to side to make sure he catches the scent.
- Blood discharge – This is usually the surest indicator the heat cycle has begun, with a pinkish red-colored discharge the first week that usually turns to a tannish color during the fertile period, and then changes back to a reddish color before gradually stopping altogether.
- Swollen vulva – The vulva can show some swelling, but it is quite variable, some girls hardly swell at all, while others swell up like a golf ball.
Frequency of Cycles
While many females will hold to a fairly steady schedule of coming into the season about every six months, it can vary. Some girls will only come into heat once a year while others may even come into season every four months. However, these “extra” seasons are not always fertile. Once a female has her first season, you can track subsequent cycles to determine what her natural pattern will be.