Estrus cycle of Dog

The heat cycles of the female (bitch) are caused and controlled by hormones that are produced and released by the ovaries and other glandular structures within the body. The ovaries are paired structures that become increasingly active when the animal passes through puberty, at the first heat cycle. This ranges from between five and eighteen months of age depending on the individual and the size of the animal. In the toy and small breeds, heat cycles occur as early as five months of age, while in the giant breeds, this may not occur until the animal is fourteen to eighteen months old. Typically, these cycles will occur every six to nine months throughout the life of the animal. In the very young and very old, there may be 'silent heats' with no outward signs that are detectable by the owner or sometimes even other dogs. Dogs do not undergo any form of menopause. There have been rare cases of heat cycles resulting in pregnancies at fifteen years of age.
The heat or estrus cycle of the female is divided into four different stages. There is great variation in the length of these cycles among individuals of the same breeds and among various different breeds. Additionally, the same animal may have significant variations over the course of a lifetime. It is therefore impossible to talk about the cycling of bitches using exact dates or time periods.
Proestrus: The first stage of a heat cycle is a preparatory period referred to as proestrus. This follows a period in which the reproductive system, was from all outward appearances, inactive. Proestrus typically lasts five to nine days. On the first day of proestrus, the vagina becomes swollen and a bloody discharge is soon observed. During this stage males show an interest in the female, but she will be unreceptive to them.
Estrus: The next stage is referred to as estrus. This is the active breeding phase, and will usually last from five to nine days. Bleeding from the vagina is very slight or completely absent at this point. Eggs are released from the ovary and travel down the oviduct. During estrus males will definitely be attracted to and attempt to mate with the female. The female will allow them to mount her, resulting in intercourse. In the dog, a 'tie' usually occurs in which the male and female are held together physically, with the vagina tightly enclosed around the glans penis. Ejaculation will occur and sperm cells will enter the uterus and make their way to the oviduct, where their union with the egg will result in fertilization. A tie, however, is not necessary for conception to take place. The union of the egg and sperm results in the formation of a fertilized egg, which is referred to as a zygote. This matures further, developing into an embryo and then a fetus.
Diestrus: Following estrus is the diestrus period. This extends from the time when the female dog is no longer receptive to the male to the end of pregnancy. In cycles in which a pregnancy did not occur, diestrus will last for a period of up to 80 days. In early diestrus, the embryos and their placentas attach to the wall of the uterus, from which they will derive their oxygen and nutrients.
Anestrus: Following diestrus is anestrus. This is the quiescent period between heat cycles characterized by no outward physical or behavioral signs of sexuality.