Influence of litter size and breed on variation in length of gestation in the dog

The variation in the length of gestation, the period from mating until parturition, was studied in 77 dogs of different breeds; the time for mating was determined by measuring peripheral blood progesterone levels. The mean length of gestation was 62.1 +/- 0.2 (S.E.M.) days, with a variation of 11 days. The number of pups appeared to influence the length of gestation. Length of gestation was negatively correlated (r = -0.96, P < 0.001, n = 44) with litter size in litters with 7 or fewer pups. The intra-breed variation in length of gestation in the five breeds represented by five or more bitches was 3-6 days. The mean gestation of Alsatians (60.1 +/- 0.5, n = 9) was shorter (P < 0.005) than that of the other breeds combined (62.3 +/- 0.3, n = 68). The primiparous/multiparous status of the bitch did not influence the length of gestation.