Dystocia may result from either maternal or fetal factors that prevent delivery from taking place. Uterine inertia is the
most common maternal cause of dystocia, seen when the myometrium produces only weak and infrequent contractions that fail
to expel a normal fetus through a normal birth canal. Primary uterine inertia is considered complete when gestation that has exceeded its expected length with no evidence of progression into active labor. Primary uterine inertia
is termed partial if the bitch initiates parturition and expels one or more healthy fetuses, but then subsequently fails to deliver the remaining
fetuses as a result of myometrial fatigue. Uterine inertia may also be considered secondary if myometrial failure results from prolonged attempts to expel an obstructed fetus, and persists following relief of obstruction.
Morphologic causes of dystocia are those in which an anatomic abnormality of the bitch or queen results in obstruction of
the birth canal (eg. small birth canal, pelvic fractures)
Fetal factors that may result in dystocia include malpresentations, oversize, fetal malformations, and fetal death. Some of
the commonly described malpresentations include transverse presentation, lateral or ventral flexion of the neck, anterior
presentation with flexion of one or both forelimbs, posterior presentation with retention of both hindlimbs, and simultaneous
presentation of two fetuses. It should be noted that posterior presentations are considered to be a normal variation in dogs
and cats, occurring in approximately 40% of deliveries. Fetal oversize is another potential cause of dystocia, most commonly
seen with single pup pregnancies. Fetal death is an infrequent cause of dystocia, increasing the likelihood of malpresentation
because of failure to rotate and extend the head and legs, which commonly occurs immediately prior to parturition. Fetal malformations
are another potential cause of dystocia, with anasarca (generalized subcutaneous edema), hydrocephalus, cerebral and cerebrospinal
hernias, abdominal hernias, duplications, and rib cage malformations among the more commonly noted.
Diagnosis of Dystocia
Workup of a patient that is presented for dystocia begins with a complete history and physical exam, including digital vaginal
exam. If a fetus is lodged within the birth canal, digital manipulation should be attempted. The fetus may be grasped around
the head and neck, around the pelvis, or around the proximal portions of the hind limbs, depending on fetal presentation.
Excessive traction should never be applied to a single extremity because of the ease with which these may be avulsed. With
the dam restrained in a standing position, traction is applied in a posterior-ventral direction. The fetus may be gently rocked
back and forth, and twisted diagonally to free shoulders and hips "locked" in the pelvic canal. If flexion of head or extremities
is preventing delivery, a finger may be used to extend them. One cannot overemphasize the importance of using copious amounts
of sterile lubricant during obstetrical maneuvers, applied digitally or infused around the fetus using a red rubber catheter.
Radiographs should be obtained in any animal experiencing dystocia. Radiographs are accurate for assessing the number, size,
location, and position of fetuses, as well as maternal pelvic morphology and general status of the abdomen. Fetal viability
is more difficult to assess from radiographs, unless evidence of fetal decomposition is present. Signs of decomposition include
intrafetal or intrauterine gas patterns, awkward fetal postures, collapse of the spinal column due to loss of muscular support,
and overlapping of the bones of the skull. Ultrasound may be a more useful tool for assessment of fetal viability, fetal malformations,
and fetal distress. Normal fetal heart rates have been reported at 180-245 beats per minute in dogs and up to approximately
265 bpm in cats. Deceleration of fetal heart rates to less than 180 beats per minute and the presence of fetal bowel movements
on ultrasound have been shown to correlate with severe fetal distress, and may indicate a need for rapid intervention.