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Reproduction is a complex event, and everything has to be just right for pregnancy to occur. By understanding this natural cycle, you can have a better appreciation for the miracle of canine life.
The Reproductive Stages
The reproductive cycle in dogs varies tremendously depending on breed and size of the animal. Puberty may occur as early as 6 months old in small breeds and as late as 2 years old in larger breeds. On average, you can assume that the first estrous cycle (heat cycle) will occur between 6 and 12 months of age. The duration of the estrous cycle in a particular dog is fairly consistent over time, but again, there's a great deal of variability between breeds. Some breeds will cycle once every 4 months (e.g. German shepherd) while others will cycle once every 12 months (e.g. basenji). In general, older bitches (female dogs) cycle less frequently than those younger. The estrous cycle in the dog can be divided into four stages: proestrus, estrus, diestrus and anestrus.
Proestrus: Getting Ready
Proestrus is the time when a bitch's reproductive tract is preparing for the possibility of breeding with a male dog. It lasts an average of 9 to 11 days, but can last for as little as 3 or as long as 21 days. Its onset is usually accompanied by a bloody discharge from the vagina as well as swelling of the vulva. Identifying these signs depends on how much hair your dog has at her hind end and how thoroughly she grooms herself. A bitch may become attractive to male dogs during proestrus, but she'll show little interest in the males at this stage. The walls of the bitch's reproductive tract thicken during proestrus and it's possible to collect epithelial cells (cells from the uppermost layer of the skin) from the reproductive tract of the bitch with cotton swabs to determine approximately how far along she is in her cycle.
If the epithelial cells are viewed under a microscope, their size and shape will change as the estrous cycle progresses. In the previous stage of the estrous cycle, anestrus (see below), the epithelial cells of the reproductive tract are small and round with a darkly stained nucleus. As proestrus ensues, these cells become larger, more angular and have little staining of the nucleus, while dramatically increasing in number. Under a microscope, many red blood cells and some white blood cells, which were not present in the previous stage, can be seen in proestrus. Hormone levels also change during proestrus. Although estrogen levels rise in anestrus, they remain relatively low during the early part of proestrus. As the bitch enters the latter parts of proestrus, however, estrogen levels continue to rise, reaching their peak near the end of this stage. Estrogen begins to fall as another sex hormone, progesterone, begins to rise. Progesterone remains high throughout the next two stages of the heat cycle, and is one of the hormones that can be measured to indicate when a bitch is about to ovulate and, therefore, should be bred.
Estrus is the next stage of the estrous cycle. It also lasts an average of 9 to 11 days, but can vary from 3 to 21 days. It can be defined as the time when the bitch accepts the male dog for breeding. During this stage, she sometimes exhibits a behavior called "flagging." This involves the bitch standing in one place, sometimes shuffling her feet, while at the same time, lifting her tail to allow the male to sniff her hind end.
During estrus, the vaginal discharge decreases, becoming less bloody in many dogs. None of the small round epithelial cells that were typical of early proestrus can be seen under a microscope in estrus. The progesterone level continues to rise. Near the onset of estrus, there is a very short rise in another sex hormone, luteinizing hormone (LH). Once this has peaked, ovulation usually occurs within 24 to 48 hours. The prime time to breed the dog is approximately 2 days after ovulation or 4 days after the LH surge.
Diestrus: A Delicate Time
The next stage of the heat cycle, diestrus, is considered the period when progesterone dominates the hormone scene. Diestrus lasts from 60 to 90 days in a dog that doesn't become pregnant or approximately 63 days in a dog that does become pregnant. As a consequence of the bitch's long exposure to progesterone when she's not pregnant, she's prone to a condition called pyometra if not spayed. Pyometra occurs when the uterus fills with bacteria and white blood cells. If untreated, pyometra can lead to fatal illness due to toxins secreted into the blood stream or the uterus actually rupturing. Thus, it's highly recommended to spay a bitch that will not be used for breeding.
In diestrus, the vaginal discharge will cease and the epithelial cells of the reproductive tract quickly appear small and round again. White blood cells can be seen in very high numbers during the first few days of diestrus. This is normal since their function is to clean up the cellular debris left behind by estrus.
Anestrus: The Final Stage
The final stage of the heat cycle is called anestrus. Anestrus lasts an average of 4 to 5 months. It's usually considered a time when the sex hormones are at very low levels, although they do continue to have small rises and declines throughout this stage. If a swab of the reproductive lining is taken and viewed under a microscope at this stage, very few epithelial cells can usually be seen. Those present will usually be small and round.