Stage I labor in dogs and cats normally lasts 12–24 hours, during which time the myometrial contractions of the uterus increase in frequency and strength and the cervix dilates. No abdominal efforts (visible contractions) are evident during stage I labor. Female dogs and cats 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 be seen. Normal vaginal discharge is clear and watery.
Normal stage II labor is marked by visible abdominal efforts, which are accompanied by myometrial contractions that culminate in the delivery of a neonate. Typically, these efforts should not last >1–2 hours between puppies or kittens, although great variation exists. The entire delivery can take 1 to >24 hours; however, normal labor is associated with shorter total delivery time and intervals (30–60 minutes) between neonates. Vaginal discharge can be clear, serous to hemorrhagic, or green (uteroverdin). Typically, female dogs and queens 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. Female dogs and cats 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.
Dystocia results from maternal factors (uterine inertia, pelvic canal anomalies), fetal factors (oversize, malposition, malposture, anomalies), or a combination of both. Clinically, uterine inertia developing after the delivery of one or more neonates (secondary inertia) is the most common cause of dystocia. The diagnosis of primary inertia requires tocodynamometry.
Dystocia can be objectively diagnosed if uterine contractility is inappropriate (generally infrequent, weak myometrial contractions) for the stage of labor, or if excessive fetal stress results from labor. Subjectively, dystocia is diagnosed if stage I labor is not initiated at term, if stage I labor is >24 hours without progression to stage II, if stage II labor does not produce a vaginal delivery within 1–2 hours, if fetal or maternal stress is excessive, if moribund or stillborn neonates are seen, or if stage II labor does not result in the completion of deliveries in a timely manner (within 4–12 hours).
Uterine and fetal monitors can be used to detect and monitor labor and fetal viability and to manage dystocia. Unresponsive uterine inertia, obstructive dystocia, aberrant uterine contractions, or progressive fetal distress without response to medical management are indications for a cesarean section.
Medical management includes administration of calcium gluconate and oxytocin based on the results of monitoring. Drugs are given only after 8–12 hours of an established contraction pattern (stage I labor) as detected by the uterine monitor and only if inertia is detected when stage II labor is anticipated. Premature administration of drugs results in suboptimal response.
Generally, the administration of calcium increases the strength of myometrial contractions, while oxytocin increases the frequency. Calcium gluconate (10% solution, 1 mL/22 kg body wt 2–4 times daily) is given when uterine contractions are ineffective or weak. It can be given SC (no more than 6 mL/site), avoiding the potential for cardiac irritability associated with IV administration. Oxytocin (0.5–2 U in dogs; 0.25–1 U in cats) is given when uterine contractions are less frequent than expected for the stage of labor. The most effective time for treatment is when uterine inertia begins to develop, before the contractions stop completely. High doses of oxytocin saturate the receptor sites and make it ineffective as a uterotonic. If fetal stress is evident (persistent or worsening bradycardia) and response to medications is poor, cesarean section is indicated.