[cesar49]

Normal gestation in the bitch is 56-58 days from the first day of diestrus or 64-66 days from the initial rise in progesterone from baseline (generally >2 ng/mL), or 58-72 days from the first instance that the bitch permitted breeding. Predicting length of gestation 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 reproductive tract (often ≥7 days). Breeding dates and conception dates do not correlate closely enough to permit 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. A drop in rectal temperature to a mean of 98.8°F (range 98.1-100.0°F) is seen in most bitches 8-24 hr before whelping. Breed, parity, and litter size can also influence gestational length. Prolonged gestation is a form of dystocia. Subtle signs of impending delivery include relaxation of the perineum, mammary engorgement, and a change in the appearance of the gravid abdomen, but these changes are not sensitive or specific. Because there is no means to effectively manage prematurely born puppies, premature intervention in the whelping process is undesirable. Unfortunately, an excessively conservative approach resulting in intrauterine fetal death is undesirable as well. Parturition in the queen occurs 64-66 days from the LH surge triggered by copulation.
Bitches typically enter stage I labor within 24 hr of a decline in serum progesterone to <2-5 ng/mL, which develops in conjunction with elevated circulating prostaglandins and is commonly associated with a transient drop in body temperature. Monitoring serial progesterone levels for impending labor is problematic due to the fact that in-house kits enabling rapid results are inherently inaccurate between 2-5 ng/mL. Commercial laboratories offering quantitative progesterone by radioimmunoassay typically have a 12-24 hr turnaround time, which is not rapid enough to make decisions about obstetric intervention. Clearly, it is beneficial to obtain information about ovulation timing, minimally by determining the onset of cytologic diestrus, for evaluating length of gestation at term.
A predictable and safe method of inducing parturition in the bitch and queen has not been determined.