Many things can impact litter size in the bitch. These include number of eggs released from the ovary during a given heat
cycle; presence of adequate numbers of normal spermatozoa to fertilize those eggs; presence of a normal reproductive tract
in the bitch to permit implantation of developing embryos, absence of disease; and persistent secretion of progesterone. This
paper will not include information about pathologic causes of lack of conception, such as uterine infection or poor semen
quality, or causes of pregnancy loss, such as canine brucellosis or premature decline in progesterone production. This paper
also will not include information about methods for enhancing number of eggs ovulated, or superovulation. That technique works
best in animals that ovulate few eggs with each heat cycle, such as cows and ewes, and has not been proven to work in species
that ovulate numerous eggs with each cycle, including bitches. Instead, this paper will focus on proven causes for decline
in litter size in bitches, to help you and your veterinarian best determine when a pathologic cause for decreasing litter
size may be present.
Dogs ovulate eggs (ova) that are immature, incapable of being fertilized. It takes, on average, 48 hours for the majority
of those ova to mature to the point where they can be fertilized. After that period, ova survive for a variable number of
days, with presumed decline in viability as they age. Therefore, optimal breeding day in dogs, for natural service or insemination
with fresh or chilled semen, is two days after ovulation. Optimal breeding day with frozen semen is three to four days after
ovulation; once thawed, spermatozoa that had been frozen live only a matter of hours so insemination must be performed when
all ova are known to have matured completely. It has been well demonstrated that litter size is optimized by breeding on optimal
breeding day in bitches. Determination of optimal breeding day is best done by measurement of progesterone in blood. Litter
size can be maximized by inseminating twice during a given heat cycle; most often breedings are accomplished two and four
days after ovulation with natural service, fresh or chilled semen, and three and four days after ovulation with frozen-thawed
Type of semen used – site of insemination
According to records from the American Kennel Club (AKC), natural service produced larger litters than did artificial insemination,
with a 15% decrease in those litters produced from fresh or chilled semen and a 25% decrease in those litters produced from
frozen-thawed semen. Scandinavian researchers reported a 30.5% decrease in litter size in bitches inseminated with frozen-thawed
semen compared to bitches inseminated with fresh semen.
In general, litter size is increased with any type of semen if it is deposited directly into the uterus; in a Scandinavian
study, litter size averaged 4.0 pups for vaginal insemination compared to 5.4 pups with intrauterine insemination. Intrauterine
insemination is particularly important when using frozen-thawed semen; in one study, transcervical intrauterine insemination
(TCI) yielded average litters of 6.0 pups while vaginal insemination yielded litters of 4.0 pups.
Intrauterine insemination can be accomplished surgically or by TCI. No good studies directly comparing these two techniques
have been published. Advantages of surgical insemination are that the reproductive tract can be observed directly and the
technique is successful in all attempts. Disadvantages of surgical insemination are the need for general anesthesia and only
one opportunity for performance of the technique during a given heat cycle. Reported average litter size with surgical insemination
of German Shepherd dogs was 7.0 pups in one study. Advantages of TCI include lack of anesthesia and ability to perform the
procedure multiple times during a given heat. The primary disadvantage is that the technique is not possible in all dogs.
Dr. Marian Wilson, who developed the TCI technique, reported an average litter size of 5.0 pups from TCI of 46 bitches of
Breed variation in litter size has been well documented in the veterinary literature. Average litter sizes reported for small,
medium, large and giant breeds are 3.9 pups, 5.7 pups, 5.9 pups, and 6.1 pups, respectively. A review of a large number of
litters for the 15 most popular breeds registered in the AKC demonstrated this, as shown in the following table and graph.