dozens of ova, of which usually less than 20 are fertilized and remain viable. The number of embryos that survive, implant in the uterus, and grow to become puppies is only a tiny fraction of the potential.
Embryonic Implantation
Puppies’ lives begin on the road as the microscopic embryos travel along the oviducts and into the horns of the uterus over a period of eight to 12 days after breeding. Embryos finally implant (attach) in the highly vascular wall of the uterus about 17 or 18 days after mating. The number of embryos attached in each horn is not necessarily equal. In cesarean sections I commonly found one uterine horn to contain only one fetus or sometimes none, and half a dozen or more fetuses in the other. This has no known significance, and is mentioned only as a point of interest. Embryos do not normally attach in the body of the uterus.
Breeding Time
Since ovulation takes place a day or two after the bitch will first accept a male, it seems likely that a breeding program will be highly effective if we breed a bitch as soon as she will receive a male, then repeat the breeding two days later. Generally, conception is more likely following the earliest possible normal breeding.
Conception success depends on a bitch’s attitude, her general health, and how well her hormones are working. Most of us who have kept intact females can attest to the fact that when “turned out,” most bitches do quite well on their own. Nature’s timing seems to be highly effective.
Whereas males are usually indiscriminate in their choice of mates, many times females will select their partners with discretion, allowing only a particular male or two the privilege of mating with her. Unfortunately, they do not always make the most prudent choices when left to their own devices.
Puppies of a single litter may be sired by more than one male. That usually occurs if the bitch is allowed to cruise the streets to choose her own mate(s), and is bred by several males during her receptive period (a practice to be discouraged at all costs).
Artificially Induced Estrus
Using female hormones, it is fairly simple to chemically induce outward signs of normal estrus in bitches. Artificially producing a fertile estrus is quite another project. A great many research trials conducted over several decades have yielded inconclusive results. Some trials reported relatively consistent success using oral estrogens daily. Some of that work is difficult or impossible to reproduce in practice.
Other researchers in the past had relatively good and reproducible success using a series of injections of estrogens, lutenizing hormone (LH), and follicle stimulating hormone (FSH). Unfortunately, LH hormone is no longer commercially available.
Breeding chart, using averages for the species.
Diestrus (Formerly Called Metestrus)
The third stage of canine reproductive cycles is known as diestrus. In literature from prior years, this phase was called metestrus, and had a slightly different academic definition. For practical purposes, metestrus and diestrus are synonymous. Diestrus lasts about 60 days. It is currently defined as the period extending from the last day a bitch will accept a male until the termination of pregnancy. If she is not mated, a female’s outward appearance during diestrus is indistinguishable from anestrus (see below).
Cavalier King Charles Spaniel pups mature quickly and are extremely energetic.
Because ovulation is spontaneous in the bitch, it is not signified by outward signs, receptivity, or male interest. Once ovulation occurs, the follicles from which eggs were discharged begin to recede. Diestrus is under the influence of the hormone progesterone, which originates from those corpora lutea (plural of corpus luteum). Progesterone is often called the hormone of pregnancy, and is responsible for the maintenance of the uterus and the fetuses implanted and growing therein.
Anestrus
For about 100 to 150 days, bitches’ reproductive systems are in a quiet stage of the estrous cycle known as anestrus. One reference lists the anestrus period as two to nine months. As in other phases of the cycle, breed, condition, nutrition, and size influence the duration of anestrus. During that period, bitches exhibit no signs of heat. Their external genitalia (vulvas) remain normal in size and color, and there is no vaginal discharge. Their temperaments and dispositions are normal, and they show no sexual interest in males. It is the period of uterine rest, repair, and stabilization.
The only reproductive hormone found in the bloodstream in appreciable quantities during anestrus is estrogen. Although some consider anestrus to be a period of reproductive inactivity, the animal’s pituitary gland and ovaries are actively preparing for the next proestrus phase to begin.
As you can see, proestrus accounts for about nine days, and estrus for another nine; diestrus accounts for two months, and anestrus four or more months. Most bitches (except Basenjis) complete more or less two estrus cycles each year. The age at which cycles commence depend less on climatic conditions and geographical variations, and are more influenced by nutrition, health, and physical size.
Interestrous Period
The time interval between periods of receptivity of canine females is termed the interestrous period. It varies greatly between breeds and between individuals within a breed. One lengthy study found the interestrous interval in German Shepherds to be as short as 149 days, and in Boston Terriers to be as long as 242 days. Bassets, Beagles, Cockers, Pekingese, and Poodles fell in between.
German Shepherds and Rottweilers sometimes have estrous cycles of four months or less. Bitches that demonstrate interestrous intervals of less than 130 days may not have ample organ repair and preparation time during anestrus. Infertility is occasionally seen in those animals, and hormone treatment can often be used to lengthen their interestrous periods.
Age
Canine estrous or heat cycles begin at puberty and occur throughout intact bitches’ lives until death. Spaying (removing the uterus and ovaries) ends the estrous cycles. Puberty is reached at varying ages—at five months in some toy breeds, or as late as 30 months in some giant breed individuals.
Many canine research studies use Beagles as models. In that breed, puberty is typically reached at about ten months, and it directly correlates with growth plane maturity. Since smaller breeds mature at a younger age than large breeds, it usually follows that tiny breeds experience puberty at an earlier age than giants.
We ordinarily speak of canine estrous cycles as six-month periods, but rarely do they take exactly six months to complete. The lengths of cycles not only vary between breeds, but considerable differences are common between females of the same breed. An individual bitch’s cycles are usually very predictable from year to year, but they may also vary due to health and condition influences. Some authorities speculate that the duration of lactation (length of time she nurses her puppies) may modify a particular bitch’s cycle as well.
TIP
If your bitch has not shown evidence of a heat period by two years of age, consult your veterinarian or a canine reproductive
