Dystocia: Medical and surgical management (Proceedings)
Although many bitches and queens deliver in the home or kennel/cattery setting without difficulty, requests for veterinary obstetrical assistance are becoming more common. The increased financial and emotional value of stud dogs, brood bitches, toms, queens and their offspring to the pet fancy makes the preventable loss of even one neonate undesirable. Breeding colonies in academic, scientific and industrial facilities need to maximize neonatal survival for financial and ethical reasons. Veterinary involvement in canine and feline obstetrics has several goals: to increase live births (minimizing stillbirths resulting from the difficulties in the birth process), to minimize morbidity and mortality in the dam, and to promote increased survival of neonates during the first week of life. Neonatal survival is directly related to the quality of labor. Optimal management of whelping/queening requires an understanding of normal labor and delivery in the bitch and queen, as well as the clinical ability to detect abnormalities in the birthing process.
Dystocia is defined as difficulty in the normal vaginal delivery of a neonate from the uterus. Dystocia must be diagnosed in a timely fashion for medical or surgical intervention to improve outcome. Additionally, the etiology of dystocia must be identified for the best therapeutic decisions to be made.
Clinicians are commonly asked to ascertain if a bitch or queen is at term pregnancy, ready chronologically to deliver a litter, and then to intervene if labor has not begun. An accurate determination of gestational length can be difficult, especially if numerous copulations occurred and no ovulation timing was performed. Prolonged gestation is a form of dystocia. Gestation in the bitch is more challenging to calculate than in the cat, because bitches are spontaneous ovulators. Normal gestation in the bitch is 56 to 58 days from the fist day of diestrus (detected by serial vaginal cytologies, defined as the first day that cytology returns to ≤50% cornified/superficial cells), 64 to 66 days from the initial rise in progesterone from baseline (generally >2ng/ml), or 58 to 72 days from the first instance that the bitch permitted breeding. Predicting gestational length without prior ovulation timing is difficult because of the disparity between estrual behavior and the actual time of conception in the bitch, and the length of time semen can remain viable in the bitch reproductive tract (often up to >7 days). Breeding dates and conception dates do not correlate closely enough to permit very accurate prediction of whelping dates. Additionally, clinical signs of term pregnancy are not specific: radiographic appearance of fetal skeletal mineralization varies at term, fetal size varies with breed and litter size, and the characteristic drop in body temperature (typically less than 99 degrees Fahrenheit) may not be detected in all bitches and varies in many. Breed, parity and litter size can also influence gestational length. Because the queen is an induced ovulator (ovulation follows coitus by 24-36 hours), gestational length can be predicted more accurately from breeding dates, assuming copulation provided adequate coital stimulation for the LH surge and subsequent ovulation, and a limited number of copulations were permitted. The gestational length of queens ranges from 52-74 days from the first to last breeding. The mean gestational length is 65-66 days. Because of the poor outcome with the delivery of premature puppies and kittens, elective intervention is best delayed until stage I labor has begun, or prolonged gestation confirmed.
Labor and Delivery
Bitches typically enter stage I labor within 24 hours of a decline in serum progesterone to below 2-5 ng/ml, which occurs in conjunction with elevated circulating prostaglandins and is commonly associated with a transient drop in body temperature, usually to <100 degrees Fahrenheit. Queens typically enter stage I labor 24 hours after serum progesterone levels fall to less than 2 ng/ml. Monitoring serial progesterone levels for impending labor is problematic due to the fact that in house canine kits enabling rapid results are inherently less accurate between 2-5 ng/ml, and a rapid decline in progesterone levels can occur over a period of a few hours. Commercial laboratories offering quantitative progesterone by chemiluminescence typically have a 12 to 24 hour turn around time, which is not rapid enough to enable decisions about an immediate indication for obstetrical intervention.
Stage I labor in the bitch normally lasts from 12 to 24 hours, during which time the uterus has myometrial contractions of increasing frequency and strength, associated with cervical dilation. No abdominal effort (visible external contractions) is evident during stage I labor. Bitches may exhibit changes in disposition and behavior during stage I labor, becoming reclusive, restless, and nesting intermittently, often refusing to eat and sometimes vomiting. Panting and trembling may occur. Vaginal discharge is clear and watery.
Normal stage II labor in the bitch is defined to begin when external abdominal efforts can be seen, accompanying myometrial contractions to culminate in the delivery of a neonate. Presentation of the fetus at the cervix triggers the Ferguson reflex, promoting the release of endogenous oxytocin from the hypothalamus. Typically, these efforts should not last longer than1-2 hours between puppies, although great variation exists. The entire delivery can take between 1 to >24 hours, however normal labor is associated with shorter total delivery time and shorter intervals between neonatal births. Vaginal discharge can be clear, serous to hemorrhagic, or green (uteroverdin). Typically bitches continue to nest between deliveries, and may nurse and groom neonates intermittently. Anorexia, panting and trembling are common.
Stage III labor is defined as the delivery of the placenta. Bitches typically vacillate between stages II and III of labor until the delivery is complete. During normal labor, all fetuses and placentae are delivered vaginally, although they may not be delivered together in every instance.
The stages of labor in the queen can be similarly defined. Stage 1 labor in the queen is reported to last 4-24 hours and stages II and III from 2 to 72 hours, although completion of delivery of neonates within 24 hours is expected with normal queening.
Dystocia results from maternal factors (uterine inertia, pelvic canal anomalies, intrapartum compromise), fetal factors (oversize, malposition, malposture, anatomic anomalies) or a combination of both. For effective management, the recognition of dystocia must be made in a timely manner, and identification of etiologic factors made correctly.
Uterine inertia is the most common cause of dystocia. Primary uterine inertia results in the failure of delivery of any neonates at term, and is thought to be multifactorial, including metabolic defects at the cellular level. An intrinsic failure to establish a functional, progressive level of myometrial contractility occurs. A genetic component may be present. Secondary uterine inertia results in the cessation of labor once initiated, and consequential failure to deliver the entire litter. Secondary inertia can result from metabolic or anatomic (obstructive) causes, and is also thought to have a genetic component. Birth canal abnormalities such as vaginal strictures, stenosis from previous pelvic trauma or particular breed conformation, and intravaginal or intrauterine masses can cause obstructive dystocia. In most cases, canal abnormalities can be detected in the pre breeding examination, and resolved or avoided by elective cesarean section. Causes of intrapartum compromise rendering the dam unable to complete delivery include metabolic abnormalities such as hypocalcemia and hypoglycemia, systemic inflammatory reaction, sepsis, and hypotension (due to hemorrhage or shock).
Fetal factors contributing to dystocia most commonly involve mismatch of fetal and maternal size, fetal anomalies and fetal malposition and/or malposture. Prolonged gestation with small litter size can cause dystocia due to an oversized fetus(es). Fetal anomalies such as hydrocephalus and anasarca similarly can cause dystocia. Fetal malposition (ventrum of fetus proximal to the dam's dorsum) and fetal malposture (flexed neck and scapulohumeral joints most commonly) promote dystocia as the fetus cannot transverse the birth canal smoothly.