Physiology and Endocrinology
The eggs released by the bitch are fertilized in the uterine tubes and move into the uterus 8 to 9 days after ovulation. The conceptus implants and the placenta begin to be formed 16 to 18 days after ovulation. High concentrations of progesterone must be present throughout pregnancy. Progesterone during pregnancy decreases uterine contractility; stimulates secretions of the endometrial glands, which presumably provide nutrients to maintain the conceptus before implantation; and stimulates mammary development. Prolactin concentrations begin to rise at midgestation. Concentration of relaxin, released from the placenta, also rises beginning at midgestation ().
The body of the bitch responds to the presence of the enlarged uterus and developing fetuses with processes designed to maintain function in the bitch while promoting development and growth of the puppies. Physiologic changes in the bitch that occur during normal pregnancy include the following:
- • Increased heart rate
- • Increased packed cell volume (% of the blood made up of red blood cells)
- • Increased oxygen consumption
- • Slower gastric emptying time
- • Increased blood flow to the kidney
Superfecundation is the presence in one litter of pups with different sires. This occurs readily in any bitch bred by more than one male. In some breeds, superfecundation is encouraged because it allows for greater genetic diversity in any given litter. This is especially useful in breeds with a small number of individuals. The American Kennel Club (AKC) does allow breeding a bitch to more than one male; genetic parentage testing of the pups is required before they can be registered ().
Superfetation occurs when a pregnant animal is bred and a second conception occurs. This has not been documented to occur in dogs, nor is it likely because dogs ovulate all ova over a 24-hour period and because spermatozoa would be unable to reach the uterine tube past developed zonary placentas. If there are pups of grossly different size in one litter, it is most likely that the small pup had an abnormal placenta or developmental abnormality.
Gestation length in the dog most commonly is reported to be 63 days. However, that gestation length is from ovulation, not from breeding. It has been demonstrated that gestation length timed from ovulation in most bitches is from 62 to 64 days. Gestation length timed from breeding can vary from 58 to 71 days. Remember that bitches will stand to be bred for a large window of time around ovulation and that spermatozoa may live for more than a week after introduction into the bitch’s reproductive tract. Therefore if a bitch is bred very early and does not conceive until well after breeding, she will have an apparent prolonged gestation. Conversely, if a bitch is bred late and conceives very soon after breeding, she will have a shortened gestation.
Puppies do not lay down surfactant in their lungs until within days of term. Without surfactant their lungs are incapable of normal function and pups born without sufficient surfactant either die of lack of oxygen or develop respiratory disease. Little research has been done to determine exactly how early in pregnancy puppies can be taken by cesarean section and be viable, nor has research been done evaluating the usefulness of cortisone in promoting formation of surfactant, as is done in humans. Elective cesarean section should not be performed prior to 61 days from ovulation unless the bitch’s life is at stake.
Litter size is best assessed by radiographs late in pregnancy. Several factors determine litter size in dogs. In general, larger breeds produce larger litters than do small breeds. In one report of 728,271 litters registered with the AKC from 15 breeds, the largest litters on average were from Labrador retrievers and golden retrievers and the smallest litters on average were from poodles, Pomeranians, Chihuahuas, and Yorkshire terriers. Assuming the bitch is healthy and the semen quality is normal, improper breeding management is a common cause of small litter size. Bitches bred too early have small litters because the aged spermatozoa are incapable of fertilizing all the eggs present. Bitches bred too late have few viable eggs left that can be fertilized. Optimal breeding day for good litter size is 2 days after ovulation. Age of the bitch is a factor; litter size is affected negatively when the bitch has her fifth litter and for subsequent litters, which in most bitches means when she is 7 years of age or older. Hypothyroidism has been claimed to have an effect on litter size in affected bitches.
I believe that genetic selection for other traits in a line (conformation, coat color or consistency, temperament, etc.) may be reflected in small litter size. Although it has been well demonstrated in other species, most notably in the pig, that litter size is genetically controlled and can be selected for, most breeders do not consciously select for reproductive performance in their animals and may unconsciously select against it.
Small litter size is associated with larger size of the pups born. Puppies will grow to fill the space available to them. Single pups may become too large to pass through the birth canal and cause dystocia. Single puppies also may be incapable of inducing the hormonal changes required to initiate labor in the bitch, causing prolonged gestation ().
Test Your Understanding
1) In an effort to increase the gene pool of your rare breed, the Pinkyhund, you ship in frozen semen from Liechtenstein and spare no expense for breeding management and insemination. You want to know as soon as possible if the bitch is pregnant and how many pups there are. What technique allows earliest detection of pregnancy? What technique is best for assessment of litter size?
Ultrasound at 24 to 25 days after ovulation is very accurate for pregnancy diagnosis. The dog may be palpably pregnant at this stage, and relaxin assay may be positive; but accuracy is best with abdominal ultrasound. Radiographs (x-rays) late in gestation are the best technique for assessment of litter size.
2) Your 2-year-old bitch is diagnosed pregnant by abdominal palpation 30 days after breeding. Two weeks later, you notice exudation of brown, odorless vulvar discharge. The bitch is lethargic and inappetent. You take her to your veterinarian. On physical examination, the bitch has a normal rectal temperature and her abdomen is not painful. She is slightly anemic. Her white blood cell number is normal. Ultrasound of her abdomen reveals four live pups with normal heart rates and two nonviable pups. What are possible causes of pregnancy loss at this stage in gestation? What other tests can be run to definitively diagnose the problem? How can she be treated?
Possible causes for pregnancy loss include brucellosis, canine herpesvirus, bacterial infection, hypoluteoidism, and trauma. Tests that can be run include serologic testing for brucellosis (), serologic or PCR testing for canine herpesvirus, culture of the vulvar discharge, and measurement of progesterone in serum. As for treatment, there is no way to stop the impending abortion if it is due to brucellosis or canine herpesvirus. Dead pups should be allowed to pass; if they are artificially maintained in the uterus through medical intervention, the bitch will be predisposed to metritis (uterine infection) or septicemia (bacteria in the bloodstream and subsequent body-wide infection). If progesterone concentration in blood is low at this time, I would be hesitant to supplement her because that extra progesterone will keep her uterus from contracting and expelling the dead pups. If she tests negative for brucellosis, I would put her on antibiotics based on the culture and sensitivity and recheck ultrasound every couple of days to keep an eye on the pups. Anemia is normal in late pregnancy unless it is extreme.
Selections from the book: “The Dog Breeder’s Guide to Successful Breeding and Health Management”, 2006.