C. Scott Bailey, DVM, MS, DACT
Consultant, Veterinary Answers
Veterinarians are often asked to perform pregnancy diagnosis and time a c-section with very little information from the owner. Often, the only information provided is the breeding-dates and occasionally even those are hard to come by. Consequently there is a need for veterinarians to be familiar different with methods of estimating gestational age. This is particularly important when an elective caesarean section is desirable. Elective c-sections can carry an excellent prognosis for maternal and fetal viability when timed correctly and may be less stressful to the bitch, puppies and attending veterinarian than waiting for a potential dystocia. Animals that are particularly good candidates are those with a history of dystocia or a c-section and animals that have small (less than 3 pups) or large (more than 8 pups) litter sizes. Dogs of certain breeds have a known predisposition to dystocia, such as Boxers, Bulldogs, Scottish Terriers, Great Danes and Bernese Mountain Dogs .
A number of factors play critical roles in the ultimate success-rate of elective c-sections, including fetal maturity, patient preparation, selection of anesthesia protocol and surgical technique as well as neonatal care of the pups. In this review we focus on only the first of these – Timing of c-section to maximize fetal maturity.
Three basic methods exist to predict parturition in the bitch:
While the easiest methods for timing involve breeding management, breeding dates provided by owners are notoriously unreliable. Parturition may occur anywhere from 57 to 72 days after a single observed breeding . On the contrary, the easiest and most accurate way to predict whelping is to diagnose or estimate the time of LH surge. Bitches reliably whelp 64-66 days post LH surge [2,3], which can be diagnosed by repeated LH assay (every 12 hours due to the short duration of the LH surge in the bitch). LH-peak may also be estimated by observing serum progesterone levels that achieve 2-3ng/ml and continue to rise thereafter [3,8]. Shortly after this period, vaginal cytology may be used to diagnose the onset of diestrus, occurring approximately 51-60 days before whelping .
Thereafter a variety of measures represent guides to estimate gestational stage within 2-3 days [4-6]. Further, equations have been developed to calculate gestational age in a variety of breeds [6,7].
A brief summary of useful ultrasonographic and radiographic markers of gestational age is listed below:
Ultrasonographic examination [5-8]
The fetal heartbeat is visible at approximately 22-26 days.
Limbuds, fetal movement and a fluid filled stomach may be seen on day 29, 30 and 33.
Fetal length exceeds chorionic width at approximately day 42.
Radiographic examination [5,7,9]
Pregnancy can first be reliably diagnosed radiographically day 45-48 post LH surge. More specific information is also available describing the appearance of specific structures in relation to whelping.
The scapula, humerus and femur are first detectable 17 days (15-18) prepartum.
The pelvis and 13 pairs of ribs are visible 11 days (9-13) prepartum.
Teeth are visible 4 days (3-8) prepartum.
During the final days of gestation, cortisol is produced and released from the maturing fetal adrenal gland in response to space-constraint and other physiologic stressors. This results in production of prostaglandin F2α in the placenta and endometrium, which in turn induces luteolysis and starts the cascade of events that ultimately result in fetal expulsion . At the same time the cortisol also has critical effects on the fetus, resulting in rapid maturation of vital organ systems, including the musculoskeletal system, gastrointestinal system and lungs. Prior to these final maturation processes, puppy survival may be decreased due to weakness, poor mobility and respiratory distress after removal from the uterus. On the contrary, if these processes have occurred and the bitch experiences a dystocia, survival may also be decreased. Consequently, the goal of gestational timing should be to predict whelping accurately enough to intervene after final maturation has occurred but before the bitch is in active labor. To do this, repeated monitoring of hormone levels during the final week of gestation, in combination with fetal monitoring via ultrasound or tocometry, may improve fetal viability and prevent dystocias [11-13].
Progesterone measures below 2ng/ml indicate imminent parturition within 18-36 hours.
A temperature drop by 1-3F from previous measures occurs in 75-85% of bitches within 8-18 hours prior to parturition.
Fetal heart-rate can accurately diagnose fetal distress during late gestation [12,13]
Normal late pregnancy: 200 beats/min
Fetal Stress: 180 beats/min – Values in this range indicate readiness for parturition
Fetal distress: 150 beats/min – values of 150 or below indicate the urgent need for emergency intervention to save the puppy.
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