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The female dog's reproductive cycle is controlled by a complex waxing and waning of many hormones from both the ovaries and the pituitary gland in the brain. During estrus (the heat cycle) the high levels of estrogen make the bitch feel lusty. and they attract--by scent--the attentions of males.
Ovulation, which occurs when the ripe egg ruptures from the ovary and passes into the uterus, takes place during every cycle, whether or not breeding takes place. Fertilized eggs attach to the lining of the uterus, forrn a placenta and grow into fetuses. Unfertilized eggs pass out of the body or are absorbed. The body, not knowing if conception will occur, prepares for pregnancy with each cycle.
As ovulation occurs, the tissue in the ovary that surrounded and nurtured the egg (called the corpus luteum) begins producing a hormone called progesterone. This hormone prepares the body for pregnancy. It causes the uterine lining to prepare for the attachment of embryos, the development of milk-producing tissue in the breasts, and the mental preparation of the female for caring for her young. If conception occurs, the production of progesterone is necessary to maintain pregnancy and continues until just before delivery.
In animals that cycle monthly (including human females), this progesterone phase is short and soon goes back into another estrogen phase as new eggs ripen. In dogs, however, the female cycles only twice a year, and the nurturing stage (known medically as the luteal phase because it is dominated by the hormones from the corpus luteum) tends to be longer, even when breeding and conception have not occurred. Since the hormones are the same as if the dog were pregnant, she acts as if she is.
False pregnancy in dogs is called pseudocyesis. During a false pregnancy, bitches seem to have enlarged abdomens (caused by relaxation of the ligamentous structures), exhibit breast development and produce milk. and tend to show strange behavior, such as being more loving--or more aggressive--making a nest, or gathering toys or other inanimate objects to mother.
False pregnancies go away on their own in eight or nine weeks (the length of a normal pregnancy). During lactation (milk production) the breasts can become huge and swollen or infected, so careful observation is necessary.
At 6 years of age, your dog is too old to have puppies, but she will tend to have false pregnancies each time she cycles, which will continue throughout her life. Dogs do not go through menopause.
The older a bitch is, the more she is prone to endometriosis, a progesterone-induced change in the lining of the uterus that progresses to pyometra. Pyometra complex is a serious and potentially life-threatening infection of the uterus that usually requires emergency surgery. Because of this, the best course is to have your dog spayed before it becomes a high-risk--and more expensive--necessity.
Author(s): Wilcox, Bonnie, D.V.M.
Canine Library: Reproduction