Normal gestation in dogs is 56–58 days from the first day of diestrus, 64–66 days from the initial rise in progesterone from baseline (generally >2 ng/mL) or the LH surge, or 58–72 days from the first instance that the female permitted breeding. Parturition in the queen occurs 64–66 days from the LH surge triggered by copulation.
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 dog, 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, and 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) (37.1°C [range 36.7°-37.8°C]) is seen in most dogs 8–24 hours before whelping. Breed, parity, and litter size can also influence gestational length. 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. Likewise, an excessively conservative approach resulting in intrauterine fetal death is undesirable as well.
Dogs typically enter stage I labor within 24 hours of a decline in serum progesterone to < 2 ng/mL, which develops in conjunction with increased circulating prostaglandins and is commonly associated with a transient drop in body temperature (< 99°F [37.2°C]). Monitoring serial progesterone levels for impending labor is problematic because in-house kits enabling rapid results are inherently inaccurate between 2 and 5 ng/mL. Commercial laboratories offering quantitative progesterone by chemiluminescence typically have a 12- to 24-hour turnaround time, which is not rapid enough to make decisions about immediate obstetric intervention. Progesterone levels can drop rapidly in a matter of hours. If progesterone is < 2 ng/mL, gestation is likely at term and labor is pending. Clearly, it is beneficial to obtain information about ovulation timing, minimally by determining the onset of cytologic diestrus (the diestrual shift), to evaluate length of gestation at term. Elective cesarean sections can be performed safely at >62 days gestation; earlier, neonatal surfactant is not reliably present.
A predictable and safe way to induce successful parturition in dogs and cats has not been developed. The use of progesterone antagonists (aglepristone) is promising.