Estrus "Heat" Cycle

Intacted (non-spayed) females will typically come into estrus or "heat" twice a year. The start of their first cycle and the duration differs depending on the breed and the individual dog.

Their heat is in four stages.
  • Phase 1: Proestrus (4 - 20 days) She'll have some blood or vaginal discharge. Minor swelling of the vulva.
  • Phase 2: Estrus (5 - 13 days) Vulva is swollen and their may be a yellowish discharge. She will mate during this stage.
  • Phase 3: Diestrus (last 60-90 days) This phase is when the progesterone levels start to return to normal. A "false pregnancy" can occure during this stage.
  • Phase 4: Anestrus (2 to 3 months) of inactivity between heat cycles.

What age do they come into heat?

  • Gernally the first heat is between 5 and 14 months old.

How long does the heat cycle last?

  • The estrus cycle last 12-21 days, but it can vary greatly depending on the indivdual dog. A good indication is the swelling, discharge, and dog are all back to normal (provided not bred).

When will they come into heat again?

  • A female dogs comes into season or heat, on average every 6 months, this can vary from dog to dog, year to year, and even cycle to cycle.

Estrus Cycle Problems & Concerns

What causes irregular heat cycles?

Female came into heat sooner than normal:
  • Some breeders have noted that introducing another female into the house hold may disrupt the other's timing. One comes into heat and brings in the others in the household. If one goes early to match the new dog, she may not be fertial. Another "normal" cycle should follow.
  • Quarerly heats may also be because the bitch is not fertile the prevoius season. See Shortened interestrous Intervals below
  • Follicuular cysts (produce estrogen)
  • Granulosa cells tumor (produce estrogens, often lead to prolonged heats)
  • Uterine Disease: Cystic endometrial hyperplasia can cause an early return to estrus.

Taking longer to come into heat that normal:
  • Interestrus tends to become longer when bitches are over 8 (sooner for larger dogs)
  • Hypothyroid (low thyroid)
  • Hyperadrenocorticism (elevated cortisol in the blood)
  • Hyperprolactinemia
  • Taking hormonal medication
  • Polycystic Overian Cysts (PCOS) can cause irregular or a lack a menstrual period

The time between heats is called the Interestrus Interval.

Interestrus Interval

Interestrus consists of the Diestrus and Anestrus Phase. This is typially about 6 months. Variations in the interestrus interval is not always abnoral. Once the heat cycle is established, most individuals will maintain a fairly consistent cycle throught their lives. It can vary from year to year, cycle to cycle. There are a few areas of concern to look out for. An irregular cycle at times, can point to more serious problems.

The Two types of Interestrous Intervals are Shortened and Prolonged.

Shortened interestrous Intervals

  • Some females have less than 4 months between estrus cycles
  • Absence of ovulation (maybe bacause of insufficent LH production) may be followed by a shortened interestrous interval. The progesterone levels never get high enough to be recognized by the hypothalamus. This situation is believed to tell the hypothalamus to start another estrus cycle. The shortened intervals are an effect, not a cause of infertility.
  • If a bitch has a long estrus (21 days or longer) followed by shortened intervals, there is suspicion for the presence of a hypothalamic or ovarian tumor.
  • Cystic ovarian follicles might short intervals, but are more commonly found with persistent estrus.

Prolonged Interestrous Intervals

  • Intervals greather than 12 months is usually considered abnormal, although it could be normal for some dogs.
  • Interestrus tends to become longer when bitches are over 8 (sooner for larger dogs)
  • Hypothyroid (low thyroid)
  • Hyperadrenocorticism (elevated cortisol in the blood)
  • Hyperprolactinemia
  • Taking hormonal medication
  • Polycystic Overian Cysts (PCOS) can cause irregular or a lack a menstrual period

Apparent irregular interestrous intervals may be misread in females with split heats.

Split Heats

A spilt Heat occurs when a bitch grows follicles on her ovaries and you'll see physical and behavioral changes characteristic of proestrus (vulva swelling, blood and vaginal fluid discharge). However, there is no progression to estrus and she does not ovulate. After a short anestrus period of several weeks (2 - 12, 4 weeks is average) she should have a complete cycle.

Prolonged/Persistent Estrus

A heat is not considered long until it reaches 35 to 40 days. The proestrus and estrus last an average of 9 days, but can last as long as 17 and 21 respectfully. This condition is usually caused by functional follicular cysts.

Short Estrus

Females older than 6 to 8 years of age may experience erratic cycles, including short estrus. A split heat cycle should also be considered in bitches with an apparently short estrus.



**** I'm not a vet or even close to an expert on this. I've just spent a lot of time reading online trying to figure out what I can do for my dogs, and my friends dogs. Above is the best information I could find.
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