Pregnancy in dogs is supported by the production of serum progesterone (P4) from the corpora lutea (CL) during the entire gestation. Progesterone is a steroid hormone that suppresses uterine contractility, maintains cervical closure, and aids in the differentiation of the endometrium, endometrial gland secretion and placental attachment [[1], [2], [3]]. During the last third of pregnancy, serum P4 concentration slowly declines to 4–16 ng/ml, followed by a rapid decline below 2 ng/ml 48 to 24 h before parturition. This final decrease in serum P4 is believed to be induced by the release of prostaglandin F2alpha (PGF2a) from the endometrium in response to increasing concentrations of fetal cortisol [4,5]. Thus, a serum P4 concentration lower than 2 ng/ml at term is associated with fetal readiness for birth [3,6,7]. However, it has been proposed that in bitches carrying small litters or singletons this luteolytic mechanism may not be efficient due to insufficient release of fetal cortisol, although to the authors’ knowledge, no association between the prepartum serum P4 decline and litter size has been reported in the literature.

Because of its predictive value for fetal readiness for birth, measuring serum P4 concentration is a useful tool for timing of elective C-sections. Elective C-section in dogs is a safe and effective procedure in cases that justify early intervention, decreasing risks to the dam and fetuses associated with dystocia [8]. In most cases, the candidates for this procedure are bitches with high-risk gestations including brachycephalic breeds prone to feto-maternal disproportion, previous history of dystocia, large litters or singletons, and nulliparous bitches aged six years or older [9,10]. Owners that do not have access to after-hours emergency clinics may also choose to perform elective C-sections. Accurate prediction of parturition date and fetal readiness for birth is fundamental to managing and planning an elective C-section to increase neonatal survival.

Apart from P4 measurement, multiple parameters are used in clinical practice to predict canine fetal readiness for birth and to determine when it is safe to perform elective C-sections. When the breeding history is known, C-sections can be planned 65 ± 2 days from the pre-ovulatory LH peak or 63 ± 2 days from ovulation date [11]. The decline in serum P4 is immediately followed by a transient decrease in rectal temperature, which is often monitored by breeders to identify impending parturition [6]. Confirmation of final fetal organ development and maturation can be assessed with transabdominal ultrasound, being renal development and intestinal differentiation and peristalsis the most reliable ultrasonographic indicators of fetal readiness for birth [12,13]. Fetal heart rate (FHR) variations exceeding 30 % are predictive of parturition within 12 h with 88 % sensitivity. Evaluation of FHR also allows to detect fetal stress [14]. Lastly, behavioral cues such as restlessness, nesting, digging, social withdraw or panting are indicative of the beginning of labor [15,16]. Many of these parameters are particularly valuable in cases where serum P4 levels have not yet dropped to the expected pre-labor range, providing critical guidance for safe and timely clinical intervention. These parameters may also be critical when evaluating singleton or small litters, since the predictive value of serum P4 concentration in these cases has been questioned and there are no studies evaluating the effect of litter size on the prepartum serum P4 decline. The aim of this retrospective study was to determine the effect of litter size on serum P4 concentrations during late pregnancy in dogs undergoing C-section. It was hypothesized that maternal serum P4 concentration does not decrease below 2 ng/ml in bitches carrying one or two puppies.