Canine pregnancy and parturition.

In contrast to the large variation in canine gestation lengths based on mating-to-whelping intervals (56 to 70 days), gestation length based on intervals from the preovulatory LH surge to whelping shows little variation (64 to 66 days). By considering the preovulatory LH surge as the central endocrine event of the fertile cycle, various events can be reasonably timed, including ovulation on day 2, oocyte maturation on day 4, reduced fertility after day 6, implantation around day 17, and development of fetal radiopacity after day 45. Throughout pregnancy, gestation is dependent on ovarian progesterone secretion and, thus, on pituitary LH and prolactin for luteotrophic support. Because prostaglandin F2 alpha is luteolytic in the bitch, it may be involved in the luteolysis observed immediately prepartum in association with rises in maternal cortisol and prolactin levels.