Normal gestation in bitches 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 luteinizing hormone (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. Ultrasonographic evaluation of renal maturation and bowel layering and motility can confirm term pregnancy. Appearance of mineralization of fetal dentition occurs at term (see fetal dentition radiograph).
A drop in rectal temperature to a mean of 37.1°C; range 36.7–37.8°C (98.8°F; range 98.1–100°F), occurs in most dogs 8–24 hours before whelping, secondary to the decline in progesterone associated with labor.
Pearls & Pitfalls
|
Breed, parity, and litter size can also influence gestational length.
Subtle clinical signs of impending delivery include relaxation of the perineum, mammary engorgement, and a change in appearance of the gravid abdomen; however, these changes are not sensitive or specific. Because there is no means to effectively manage prematurely born puppies or kittens, premature intervention in the whelping/queening 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 concentration to < 2 ng/mL, which develops in conjunction with increased circulating prostaglandins and is commonly associated with a transient drop in body temperature (< 37.2°C [99°F]).
Monitoring serial progesterone concentrations for impending labor is time-sensitive. In-house kits enabling rapid results are ideal; 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 obstetrical intervention. Progesterone concentrations can drop rapidly in a matter of hours. If the progesterone concentration is < 2 ng/mL, gestation is likely at term, and labor is pending. Prior validation of in-house kits with chemiluminescence is ideal.
Clearly, it is beneficial to obtain information about ovulation timing, at minimum by determining the onset of cytological 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.
Administering aglepristone (10–15 mg/kg, SC, repeated in 24 hours) can improve fetal maturation, surfactant availability, and functional luteolysis before an elective cesarean section.
A predictable and safe way to induce successful parturition in dogs and cats has not been developed.
