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The variation in the duration of gestation, defined as the period from mating to parturition, was studied in 113 bitches of six breeds (31 Dobermans, 31 Labrador retrievers, 14 German shepherds, 13 Bernese mountain dogs, 12 golden retrievers and 12 West Highland white terriers). The bitches were mated once on the optimal day for mating, which was established by measuring the plasma concentration of progesterone. The duration of gestation varied between 58 and 65 days with a mean of 61.4 +/- 1.5 days (+/- SD). The mean duration of gestation in West Highland white terriers (62.8 +/- 1.2 days) was significantly longer than that of German shepherds (60.4 +/- 1.7; P < 0.001), Labrador retrievers (60.9 +/- 1.5; P < 0.001) and Dobermans (61.4 +/- 1.0; P < 0.025). The variation in the duration of gestation in any one of the six breeds was low, ranging from 4 to 7 days, or less when litters of one pup (n = 4) were excluded. The number of pups per litter varied between one and 15 with a median of eight pups. In the total population, the duration of gestation was negatively correlated with litter size (r = -0.73; P = 0.005) for litters (n = 112) containing < or = 13 pups. However, within each breed multiple regression analysis failed to show any influence of litter size on the duration of gestation. The median litter size of different breeds and the mean duration of gestation were negatively correlated (r = -0.78; P = 0.03; n = 6). Therefore, although it was not conclusively proven, the results indicate that the breed is a major determinant of the duration of gestation in bitches and that the influence of breed may be ascribed to breed-related differences in litter size.