Causes of Dystocia
Dystocia can be caused by a number of things, including abnormalities attributable to the mother, to the puppies or to both. The basic causes typically involve some physical obstruction of the birth canal as it passes through the pelvis, or some type of uterine inertia where the dam’s uterine muscles do not contract forcefully enough to push the puppies out. Uterine inertia in dogs can be either primary, which refers to insufficient stimulation to start uterine contractions, or secondary, which refers to uterine muscle fatigue after the bitch has gone through a period of prolonged labor.
One of the most common causes of prolonged labor is large fetal size. The size of each puppy is determined by the genetics of the sire and dam. If an unusually small female is bred to a large male, the puppies are likely to be too large for the mother to deliver normally. Dystocia from physical blockage can also be caused by abnormal positioning of the puppies in the uterus or in the birth canal. Sometimes, the mother may have an anatomical stricture, which is an abnormal narrowing of the birth canal that prevents the puppies from being delivered naturally.
Other common physical causes of dystocia are a mismatch between the size of the puppies’ heads and shoulders and the maternal birth canal, and an abnormally steep angle of the entrance of the uterus into the pelvic canal from the mother’s abdomen. These conditions are seen most commonly in brachycephalic breeds. Brachycephalic breeds are those with broad foreheads, short muzzles and flat faces, such as the Pug, Bulldog, Boston Terrier, Boxer and French Bulldog, among others. Uterine torsion or tearing, metabolic abnormalities in the dam and malformed puppies can all contribute to canine dystocia, as well.
A number of other things can cause or contribute to dystocia in dogs. These include:
Large litter size
Poor nutrition for the dam during pregnancy
Poor overall health of the dam
Hereditary small pelvic canal diameter in the bitch
Age of the mother (very young or very old)
Congenital abnormalities causing unusually large fetal size
Vaginal canal abnormalities (strictures)
Pelvic bone abnormalities (old fractures that have healed but still reduce the size of the pelvic canal)
Interestingly, bitches that have had a cesarean section do not seem to have an increased risk of developing dystocia when delivering subsequent litters.
Prevention of Dystocia
Bitches that are intended to be used as part of a breeding program should have a thorough pre-breeding physical evaluation conducted by a veterinarian knowledgeable about canine reproduction. This can help to identify anatomical abnormalities or other conditions that may adversely affect the bitch’s ability to carry her litter to full term or to deliver it naturally. Many breeders will have abdominal radiographs (X-rays) taken of their pregnant bitch at roughly 50 to 55 days after breeding to get an estimate of the number of puppies to be expected in the litter. By this time, the puppies’ skeletons are typically sufficiently formed that a veterinarian can count them fairly accurately on the X-ray film. It is not a completely reliable count in every case. However, if the veterinarian counts at least 5 puppies on prenatal radiographs and the bitch only whelps 3 after many hours of labor, the owner will know that there are probably at least 2 more puppies – and maybe more – still in the uterus that need to be delivered either vaginally or surgically.
Dystocia is more commonly seen in female dogs than in cats.