Heat period in dog: Everything you need to know about the reproductive cycle

Author: Dr. Buteață Andra – Pet Stuff Hospital

The heat period in dogs refers to the time interval during which the female exhibits interest in mating, a period in which fertilization can occur. This period is known as estrus and occurs once every 6-12 months, depending on the breed and individual characteristics of each dog. It is important for owners to be attentive during this period and avoid unwanted matings if they do not intend to breed.

Read on to understand the behavioral and emotional changes in dogs during this period, how to prevent risks and health problems associated with this period, and how to make an informed decision about reproductive control.

Sexual Cycle (estrous cycle, gametogenesis)

With the onset of sexual maturity until the physiological cessation of sexual function, the reproductive system functions rhythmically and cyclically. The sexual cycle involves a series of phenomena and processes that result in the release of one or more eggs suitable for fertilization.

The duration of each sexual cycle and its phases depends on the species, breed, climate, diet, and individual.

In terms of the frequency of sexual cycles, females are divided into:

  • Monoestrous – have one heat cycle per year (wild animals);
  • Diestrus – have two heat cycles per year (bitches);
  • Polyestrous – have multiple heat cycles per year (mare, cow, buffalo, sheep, sow, cat). In this group, mares, sheep, and cats are seasonal polyestrous, while cows and sows are continuous polyestrous.

The evolution of a sexual cycle recognizes two main functional stages: the follicular stage and the luteal stage. The follicular stage corresponds to estrogenic activity of the ovary, specifically the ovarian follicles, and the luteal stage is dominated by the action of progesterone secreted by the corpus luteum. The two main stages are separated by ovulation (which can be spontaneous or induced). Between the follicular and luteal stages, there is an intervening stage of balance, characterized by the involution of the corpus luteum and sexual rest, known as diestrus in continuous polyestrous females and anestrus in seasonal diestrus females (physiological anestrus: before puberty, seasonal, post-partum, after the physiological cessation of sexual function; pathological anestrus: due to uterine conditions).

Within each stage, the following phases are distinguished:

  • Follicular stage – includes the proestrus and estrus phases;
  • Luteal stage – includes the metestrus and anestrus phases (hormonal rebalancing phase).

Ethological Changes (Behavioral)

In addition to changes in the genital tract, behavioral changes also occur during the sexual cycle.

During proestrus, the female shows increased interest in the male but does not accept mating. Libido (mating desire) manifests during estrus, a period in which the female accepts mating.

During the luteal phase (metestrus and anestrus), animal behavior is calm (quieting phase).

A bitch is a diestrus female with a sexual cycle duration of about 6 months. Large and giant-sized females are often monoestrous, with a cycle duration of 10-12 months. In conditions where the female is kept indoors, at a relatively constant temperature and brightness throughout the year, the two cycles no longer manifest only in spring and autumn but can occur at any time throughout the calendar year.

Proestrus lasts for 4-10 days (sometimes up to 15 days) and is dominated by vaginal bleeding and vulvar swelling (thicker vulva). The female attracts males but refuses mating.

Estrus lasts for 8-14 days and is characterized by the desire for mating. The female adopts a standing position and raises her tail to the side. She has pink-reddish vaginal secretions, later becoming pale pink with a mucous appearance. Importantly, unlike all other domestic females where the term “heat” exclusively refers to estrus, in bitches, this term covers the entire follicular phase due to the impossibility of precisely detecting, through clinical methods, the boundary between proestrus and estrus. Ovulation occurs spontaneously, approximately halfway through estrus.

Metestrus has a relatively constant duration of about 2 months and is divided into progressive and regressive metestrus, corresponding to the period of pseudopregnancy. During this time, the female is calm and may behave like a pregnant dog (subjective/objective symptoms).

Anestrus is the most variable phase in duration, ranging from 2 to 8 months. It is the phase of the estrous cycle in which all levels of sex hormones are baseline. It is considered the optimal period for all medical or surgical interventions on the genital tract and mammary gland.

How Does a Male Know When a Female is in Heat?

When a female is in heat, vaginal and urinary secretions containing various pheromones (chemicals produced by the body) are released, which can be quickly detected by the senses of males from impressive distances. Outside this period, no pheromones are secreted.

What Happens When a Dog is in Heat?

The vulva swells, becomes congested (abundantly vascularized), clear mucous secretions appear (heat mucus), and the female may arch her body and position her tail to the side.

The female initiates courtship with the male, which may include the release of pheromones, vocalization, posture changes, increased physical activity, and urination in the presence of the male.

Testing. Vaginal cytology is a rapid, low-cost procedure that can be performed on an outpatient basis and provides information about the appearance of cells in the smear based on each phase of the sexual cycle. During estrus, keratinized, anucleated cells arranged in clusters predominate. The smear background is clear.

At What Age Can a Dog Experience Its First Heat?

Dogs can have their first heat period at the age of 6 months or up to 24 months. It is not recommended for a female to become pregnant at the age of 6 months, as she is not fully developed.

The onset of the first heat period in dogs varies. Small breeds may experience their first heat period earlier, while medium-sized or giant breeds may have their first heat period between 18-24 months.

How Long Does Pregnancy Last in Dogs?

Generally, pregnancy lasts about 9 weeks – 63 days.

Do Dogs Enter Menopause?

Dogs do not experience menopause. Their reproductive cycles differ from those of humans. Dogs will continue to go into heat throughout their lives, even in old age, but the cycles occur less frequently and may have a longer duration. Older dogs should be frequently examined by a veterinarian, especially those that have ceased to have cycles, as it could signal the presence of an underlying condition or metabolic impairment.

When the mother is older, puppies tend to be smaller, there may be more stillbirths, and whelping may involve more complications. After the age of 8, females are at risk of developing pyometra (uterine infection) that can be life-threatening.

What Should An Owner Do When The Dog is in Heat?

It is advisable not to visit dog parks or other populated areas. Spaying is not recommended during this period, as there are much more complications.

The best form of reproductive control for both dogs and cats is spaying.

Before that, a preoperative consultation by the veterinarian should be performed regarding the necessary investigations before surgery and to choose the most suitable time for sterilization, depending on the age and clinical condition of the patient.

Should I Let My Dog Have One Heat Cycle/Litter Before Sterilization?

There are no valid reasons to let a dog have a litter before sterilization. Recent research has shown that some large breed dogs (Labrador Retriever, Golden Retriever, German Shepherd) may medically benefit from delaying the sterilization operation until after the first heat cycle. However, the consensus at this time is that sterilization will increase a dog’s lifespan.

A myth suggests that females will become more sociable and friendly if they have puppies. This is not true and contributes even more to the serious problem of dog overpopulation.