Is the pre-partum serum progesterone decline associated with litter size in dogs? A retrospective study

The accuracy of the prediction of fetal readiness for birth is essential for a proper timing of cesarean sections (C-section) and survival rate of the neonates. In clinical practice, serum progesterone (P4) concentration has been used to determine when to safely intervene. Nevertheless, it has been proposed that in small litters or singletons the decline in prepartum serum P4 concentration could be affected due to insufficient fetal cortisol release, affecting the luteolytic cascade. This retrospective study aimed to determine the effect of litter size on pre-partum serum P4 concentrations in dogs undergoing C-section. Fifty-two medical records were reviewed from pregnant bitches presented for C-section (2014-2024). Collected data included age, breed (size and type), breeding method, breeding season, gestation length, pre-partum serum P4 concentration and fetal heart rates (FHRs) on the day of the C-section and up to 3 days before the procedure, newborn gender, litter size, and neonatal survival rate. The dogs were divided based on litter size: G1: one puppy (n = 10), G2: two puppies (n = 4), and G3: three or more puppies (n = 38). Gestation length at the time of surgery did not differ between dogs with and without a decline in serum P4. The frequency of bitches with a decrease serum P4 differed by litter size and 70-75 % of bitches carrying 1 or 2 puppies did not have a decrease in serum P4 at time of the surgery. Brachycephalic breeds were more likely to show a decline in serum P4. The minimum FHR on day -1 was negatively correlated with serum P4 concentration on day 0, whereas serum P4 concentration on day -1 was negatively correlated with the female ratio. Litter size was negatively correlated with survival rate and minimum FHR at day 0. In turn, the minimum FHR on day 0 was positively correlated with survival rate. These data suggest that the littler size affects the pre-partum serum P4 concentration during late pregnancy in dogs. The pre-partum serum P4 concentration cannot be used reliably to predict fetal readiness for birth in bitches carrying 1 or 2 puppy litters.