Oestrous Cycle - Dog

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Oestrous Cycle

  • Monoestrous, with no seasonal influence.
  • A single very long cycle, follwed by a long period of anoestrus.
  • Cycle about 3 times every 2 years.
  • The luteal phase is about the same length as gestation (~60 days), so pregnancy does not affect the length of the oestrous cycle. Thus, no luteolytic mechanism is required.


  • Characterised by vulval oedema, swelling and sanguinous discharge.
    • May not always be visible because some bitches will continually lick the perineum.
  • Bitch is attractive to males, but will not accept them.
  • Duration is ~9 days


  • Accepts the male
  • Vulva becomes less oedematous
  • Vulval discharge lessens and becomes more clear.
  • Duration is ~9 days
  • Ovulation usually occurs 1-2 days after the onset of oestrus.
    • Some follicles continue to ovulate 14 days later.


  • Starts when the bitch will no longer accept the dog.
  • There is dispute over the duration of this phase, with two main theories:
    • Ends when corpora lutea have regressed at 70-80 days.
    • Ends when endometrial repair is complete at 130-140 days.


  • Transitions from metoestrus to anoestrus with no external signs.
  • This is also true after parturition.
  • Lasts about 3 months before return to pro-oestrus.

Cyclic Changes in the Uterus

  • The endometrium shows considerable change during the oestrous cycle.
  • In pro-oestrus and oestrus, endometrial glands are loosely coiled with obvious lumina and a deep epithelial lining.
  • During metoestrus, the glands become larger. Lumina become smaller and the basal layer of the endometrium becomes more tortuous.
  • As the bitch reaches anoestrus, there is a reduction in the number of glands and the amount that they coil.
  • In metoestrus (~98 days after the onset of oestrus), there is desquamation of the endometrial epithelium.
  • By day 120-130, the endometrium is restored by proliferation of cells from crypts of the endometrial glands.

Cyclic Changes in the Ovaries


  • In anoestrus, the ovaries are oval and slightly flattened.
  • In a medium sized breed, they measure approximately:
    • 1.4cm pole-pole
    • 0.8 cm from the attached end to the free border
  • No follicles can be seen.
  • On section, tiny remnants of the corpora lutea of previous cycles are visible as yellow/brown spots.


  • Developing follicles are about 0.5cm in diameter.
  • They enlarge until ovulation, where their size is 0.6-1cm.


  • By the time of ovulation, the ovary is enlarged.
  • The size of the ovary depends on the number of ripe follicles present.
  • The shape of the ovary becomes irregular due to the projection of follicles from its surface.
  • The follicle wall is thick, so it is hard to distinguish between follicles and corpora lutea.
  • Prior to ovulation, the surface of follicles shows a slightly raised papule. This is about the size of a pin-head. Also, the epithelium covering this papule is brown, whereas the rest of the follicle is flesh-coloured.
  • A unique feature of the bitch follicle is the thickness of its wall.
    • Due to hypertrophy and folding of granulosa cells which is visible to the naked eye.
    • This is evidence of pre-ovulatory luteinization.
  • Spontaneous ovulation usually occurs 1-2 days after oestrus onset.
  • Most follicles rupture over a period of 48 hours.
  • The oocyte can be fertilized for up to 108 hours after ovulation.


  • At first, the corpus luteum contains a central cavity.
  • By day 10 after ovulation, the cavity becomes filled by compact luteinised cells.
  • At this time, the corpus luteum reaches its full size, measuring 0.6-1.0 cm.
  • Corpora lutea comprise the greater mass of the ovary.
  • Generally, equal amounts of corpora lutea are found on each ovary.
    • The number of foetuses in the uterine horns frequently differ from the number of corpora lutea on the ovaries of their respective sides.
    • Embryonic migration to the uterine horn of the opposite side is common.
  • On section:
    • Corpus luteum is yellow-pink
    • It remains unchanged in the non-pregnant bitch until about day 30 after ovulation.
    • It then slowly atrophies
    • During pregnancy, corpora lutea persist at their maximum size thoughout the pregnancy, but regress rapidly after parturition.

Appearance of the Ovaries on Ultrasound

  • Developing antral follicles can be seen at the beginning of pro-oestrus.
    • Appear circular and anechoic
  • In oestrus, antral follicles are increased in size, reaching a maximum diameter of 4-13mm on day 13 (day 0 is the onset of pro-oestrus).
  • From day 10 onwards, walls of the follicles become thickened due to pre-ovulatory luteinisation.
  • No evidence of follicular collapse associated with ovulation.
  • 25-30 days after the onset of pro-oestrus, the ovaries are difficult to identify.

Endocrine Changes during the Oestrous Cycle

  • The luteal phase is prolonged compared to other species, this is shown by persisting levels of progesterone in the bloodstream.
  • Progesterone levels begin to rise 60-70 hours before ovulation has occured. This is evidence of pre-ovulatory luteinisation of mature follicles.
  • The pre-ovulatory rise in progesterone may signal the bitch to accept the male.
  • Oestrogens rise rapidly before the onset of standing oestrus.
  • The LH surge rapidly follows and lasts longer than in other species.
  • Ovulation occurs 24-96 hours after the LH surge.
  • FSH levels peak at the same time as the LH surge.
  • As progesterone levels fall towards the end of metoestrus or pregnancy, prolactin increases.
  • Prolactin is a major luteotrophic hormone in the bitch.


  • Almost all bitches show some evidence of pseudopregnancy during metoestrus.
    • Covert pseudopregnancy: where the bitch is in metoestrus, but shows little or no signs.
    • Overt pseudopregnancy: where the bitch shows signs ranging from slight mammary development and lactogenesis to mock-parturition with nesting, loss of appetite, straining, emotional attachment to inanimate objects and heavy lactation.
  • There is no difference in progesterone concentrations in bitches with or without pseudopregnancy.
  • Prolactin is probably responsible for initiating the changes.