Reproductive Services

  

Canine Ovulation Timing

The estrus cycle of the bitch consists of four stages:

STAGE 1 STAGE 2 STAGE 3 STAGE 4  


Proestrus - Stage 1

Proestrus begins with the observable signs of heat, such as vulvar swelling, bloody vaginal discharge and attraction of males. Hormonally, proestrus is characterized by rising estrogen levels. 

Estrus - Stage 2

Behavioral estrus is the period of female receptability, this behavior can be characterized by “flagging” and “winking”. The vulva will soften and decrease in size, and often the discharge will change from bloody to clear or straw-colored. Hormonally, estrus levels begin with the luteinizing hormone (LH) and are characterized by declining estrogen and rising progesterone levels. Ovulation and the fertile period occur during this stage.  

Diestrus - Stage 3

Diestrus begins approximately 8 days (range 6-10 days) after the LH surge, and signifies the end of the fertile period. Progesterone levels continue to rise and remain elevated during diestrus in all normal bitches whether pregnant or not: this elevation in progesterone is necessary to maintain a pregnancy.

Anestrus - Stage 4

Anestrus is the transition period between one cycle and the next. Progesterone levels return to baseline, either abruptly just prior to whelping, or gradually in the non-pregnant bitch. The reproductive tract “rests” for several months while some hormonal changes occur to prepare the bitch for her next heat cycle.

Important Hormones

Estrogen | Luteinizing Hormone | Progesterone

Progesterone

Blood progesterone levels stay at a low baseline level during late anestrus and proestrus, they begin to rise at the time of the LH surge. Progesterone then remains elevated for two to three months. These increasing levels of progesterone together with decreasing estrogen levels cause the reduction in swelling of the reproductive tract that occurs during estrus. Increased progesterone is also necessary to maintain pregnancy. Measurement of blood levels of this hormone is consistent between bitches and is simple to perform. Therefore, serial blood samples can be performed to identify the initial rise in progesterone that coincides with the LH surge, and breeding may be based on this parameter.

Estrogen

During proestrus, serum estrogen levels rise slowly over a period of 10-14 days, peak 2 to 3 days before estrus then decline rapidly. Estrogen’s major role is to prepare the reproductive system for breeding; therefore peak levels are reached prior to ovulation. These levels in estrogen result in:

  1. Behavioral changes
  2. An increased turnover rate of vaginal epithelial cells (the cells lining the wall of the vagina). This process results in the changes seen in vaginal cytology.
  3. Progressive swelling of the female reproductive tract and bloody discharge.

Blood estrogen levels are not reliable for timing breeding. Estrogen levels vary significantly from bitch to bitch, and there is no meaningful way to standardize the results. In addition, changes in estrogen are not directly correlated to the fertile period.

Luteinizing Hormone

Normally LH is present in very small quantities. In early estrus, a significant increase in serum LH, followed by a return to baseline values occurs rapidly, usually over a 24-hour period. It is this LH surge that triggers ovulation and thus determines the fertile period of the bitch. All events subsequent to the LH surge are consistent between bitches.

The detection of a surge in release of luteinising hormone indicates impending ovulation. LH can be detected by urinary ovulation predictor kits that are performed, typically daily, around the time ovulation may be expected.

Progesterone

Blood progesterone levels stay at a low baseline level during late anestrus and proestrus, they begin to rise at the time of the LH surge. Progesterone then remains elevated for two to three months. These increasing levels of progesterone together with decreasing estrogen levels cause the reduction in swelling of the reproductive tract that occurs during estrus. Increased progesterone is also necessary to maintain pregnancy. Measurement of blood levels of this hormone is consistent between bitches and is simple to perform. Therefore, serial blood samples can be performed to identify the initial rise in progesterone that coincides with the LH surge, and breeding may be based on this parameter.

Estrogen

During proestrus, serum estrogen levels rise slowly over a period of 10-14 days, peak 2 to 3 days before estrus then decline rapidly. Estrogen’s major role is to prepare the reproductive system for breeding; therefore peak levels are reached prior to ovulation. These levels in estrogen result in:

  1. Behavioral changes
  2. An increased turnover rate of vaginal epithelial cells (the cells lining the wall of the vagina). This process results in the changes seen in vaginal cytology.
  3. Progressive swelling of the female reproductive tract and bloody discharge.

Blood estrogen levels are not reliable for timing breeding. Estrogen levels vary significantly from bitch to bitch, and there is no meaningful way to standardize the results. In addition, changes in estrogen are not directly correlated to the fertile period.

Ovulation and the Fertile Period

Ovulation is triggered by the LH surge, which causes the ovaries to release the developing ova, or eggs. Ovulation occurs 48 hours after the LH surge. The ova cannot be fertilized upon their immediate release from the ovaries. The ova require 2 to 3 days to mature before sperm penetration and fertilization can take place. Once the eggs are mature, they remain viable for approximately 2 to 3 more days before they begin to degenerate. Thus, the actual fertile period of the bitch is only 2 to 3 days long and begins 4 to 5 days after the LH surge (2-3 days after ovulation), and occurs just prior to the onset of diestrus.

Breeding Dates

In routine natural breeding or fresh AI with a normal healthy stud, sperm may be expected to live within the bitches reproductive tract 5 or more days. Thus, breeding performed a day or two before the fertile period should be still are viable at the time of peak fertility. Subsequent breedings during the fertile period will maximize sperm numbers on the bitch’s most fertile days. It is recommended to breed on days 2, 4 and 6 post LH surge if three breedings are possible. If not, perform two breedings between days 3 and 7 post LH surge.

If using chilled or frozen semen, or performing a natural breeding or fresh AI using a stud with compromised semen, it may be assumed that the longevity of the sperm cells is decreased in early breedings and will likely be wasted. Therefore, these breedings should take place during the true fertile period, days 4-7 post LH surge (2-3 days post ovulation).

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