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The normal reproductive cycle of the bitch consists of four phases that include proestrus, estrus, diestrus, and anestrus. Most breeds reach puberty and begin to cycle between 4 and 15 months of age. Generally, smaller breeds will come into heat sooner than larger breeds, although there is variation both within and among breeds. The domestic dog is a non‐seasonal breeder and typically has two estrus cycles per year.

The female undergoes several changes during the estrus cycle. During the proestrus stage, the bitch’s vulva becomes swollen and firm. There may be a slight discharge that ranges in color from clear to bloody. At this stage, males will be interested in her, although she will not be interested in them. The behavior at this time mimics play, including play bows, running together, and playful chasing. The female may briefly stand for the male and then move away. The bitch may also quickly turn and growl, snap, or bite an unwelcome suitor. Generally, her ears are held back, and her tail is tucked between her legs. While tail tucking is typically associated with fear, in this instance, it is an evasive behavior preventing intromission should a male become too insistent.

Behaviorally, estrus is the stage in which the female is receptive (allows copulatory mounting) to the male. During this time, her vulva is still enlarged, although it may be somewhat softer. The discharge may still be present and ranges from a clear to slightly serosanguinous fluid. The initial courtship behavior may still mimic play. The behavior may progress to more intense sniffing of genitalia. Instead of the female moving away, she will stand in a braced position with her back feet base wide and deviate her tail to the side if touched near her vulva. If the male is inexperienced, the female may mount him, or she may actively solicit the male by backing into him with her tail flagged to the side. An estrus bitch will urinate a small amount frequently, similar to a dog with a UTI. Dogs have a tremendous sense of smell, and the hormonal changes in the female’s urine attract males over a large area (Bradshaw and Nott 1995). This results in the intact male roaming in search of the female.

The proestrus and estrus stages can last anywhere from a couple of days to four weeks. Behaviorally, diestrus is the first day the female is no longer receptive to the male. The vulva decreases in size, and the female may display aggressive threats as she defends herself from males. The diestrus period is the time from ovulation to either parturition or anestrus.

Initially, there are very few behavioral changes between a gravid and non‐pregnant female. During the last two weeks of gestation, the pregnant bitch’s abdomen starts to enlarge, her nipples and mammary glands continue to develop, and, at times, milk may even leak from her glands. She may prefer to eat smaller portions and may also be polydipsic and polyphagic. The enlarged uterus filled with growing feti places increasingly greater pressure on the stomach. Small frequent meals allow the bitch to ingest enough nutrients to support herself and the growing feti and to produce milk. Milk production also requires an increase in water consumption. The enlarging mammary glands may cause discomfort, resulting in increased licking of the uncomfortable area. Depending on the size of her litter and her body condition, she may also move more slowly. The change in hormones that occur during impending parturition may result in some bitches becoming anorectic and having looser feces. This may result in an evacuated gastrointestinal system, allowing for a more sanitary birthing process because general abdominal pressure may result in fecal expulsion.

Bitches, whether bred or not, may build a nest and act pregnant. Some will become very destructive in their nest building and dig up or destroy bedding, furniture, or other household items. In addition, they may also drag items from other parts of the house or yard to construct their nest. Some bitches will regard certain toys or objects as puppies and bring them to the nest. Others may carry the “pup” around with them. Some individuals may become very protective of their “surrogate litter.”

Parturition occurs approximately 63 days after ovulation. Plasma progesterone drops to less than 2 ng/dl (Concannon et al. 1978), followed 10–14 hours later by a drop in rectal temperature. Twenty‐four hours prior to parturition, many bitches’ rectal temperature drops below 100° F for a short period of time and then returns to normal. During this time, the bitch may seem restless and uncomfortable, pant, and nest build. These behaviors may also occur intermittently during the last week or two prior to parturition, with an increase in frequency as parturition becomes imminent.

Anestrus is the time from parturition until the next estrus cycle. During this time, a bitch’s behavior should be similar to that of a spayed female or unneutered male.