Missed Breedings (Part 3 of 3)

The most common cause of missed breeding, as a cause of apparent infertility, is thought to be breeding at the wrong time; that is failure to deliver the semen into the oviducts 2 to 3 days post-ovulation.  The second most common cause is poor semen quality.  A third cause in the cycling bitch is abnormal estrous cycles, suggesting ovarian or uterine pathology.  Some result in failure to conceive and some result in pregnancy loss.

The “normal” estrous cycle is considered to occur every 6 months, commonly in the spring and fall.  The bitch is in proestrus for 9-12 days (range 3 to 17 days); during this time she has a firmly swollen vulva, a bloody vaginal discharge, and is not receptive to the male dog.  Progesterone levels, usually started approximately 6 days after the onset of bloody vaginal discharge, should be less than 2.0ng/ml.  Estrus is the fertile period within the estrous cycle, typically lasts for 3-5 days.  The vulva is soft but swollen, there is a clear to straw colored vaginal discharge, and the bitch is normally very receptive to the male. Progesterone levels rise from the baseline of <2.0ng/ml to 5.ng/ml at ovulation, and into the 20s and 30s ng/ml post-ovulation.  Diestrus immediately follows estrus and lasts from 56-58 days after ovulation.  Anestrus is the period between estrus cycles, typically lasting 2 to 9 months.  There is no vaginal swelling or discharge.  She is not receptive to the male and the male is not interested in her.

Abnormal Estrous Cycles

  • Failure of the bitch to cycle: for some breeds it may be normal for a bitch under the age of 24 months to no have had an estrous cycle.  This bitch should be closely evaluated to assess her external genitals.
  • Genetic intersex: there are several combinations of defects in sexual differentiation described in dogs over 2 years of age.  This can be diagnosed by karyotyping the DNA to evaluate the presence of X and Y chromosones, or by biopsying the gonads and performing histopathology to detect the presence of ovarian and/or testicular tissues.
  • Silent estrous: this is defined as ovarian activity with ovulation occurring but with no typical signs such as vaginal swelling or discharge.  This is a common cause of apparent infertility.
  • Pharmacologic interference: several drugs can block the estrous cycle.
  • Illness: bitches with many systemic illnesses may fail to cycle.  A thorough clinical history and physical examination, as well as a CBC, chemistry panel, and urinalysis should be performed to assure that the bitch is in good physical condition with no apparent underlying disorder.
  • Hormonal abnormalities: both hypothyroidism and hyperadrenocorticism are suspected of interfering with normal estrous cycles.
  • Functional ovarian cysts: luteal ovarian cysts that are secreting progesterone may cause a failure to cycle.  This may be diagnosed with progesterone testing and ultrasound.
  • Split heats: this is defined as “successive short proestrous signs, at intervals of 2 weeks to 2 months, associated with short interestrus intervals”.  Most bitches with a split heat cycle are young, stressed by travel during estrus, or Northern breeds.  They will start proestrus, but fail to have a progesterone rise that exceeds 5ng/ml.
  • Prolonged estrous cycle: generally defined as a cycle that exceeds 30 days from start of the proestrus.  Prolonged estrous cycles can be caused by a functional ovarian cyst, an ovarian tumor, the use of exogenous estrogen creams, or they can appear to be a split heat cycle.


Source: Canine Reproductive and Neonatology

Marthina L. Greer, DVM, JD