Endocrinology of pregnancy in the dog: a review.

Pregnancy regulation in the dog is not yet fully elucidated. Since plasma progesterone concentrations are similar in pregnant versus non-pregnant animals, it is a poor reflection on CL function and progesterone metabolism. Increased progesterone secretion by the CL in pregnant animals follows implantation and relaxin secretion by the feto-placental units. Progesterone is absolutely required to maintain pregnancy and no placental sources of progesterone have been identified. Pregnancy can be artificially maintained by progesterone administration. Prolactin secretion appears to be increased in response to the increase in relaxin production and occurs independent of estrogen production by the CL. The respective roles of LH, FSH and prolactin are still unclear, with considerable conflicting evidence among studies. However, it appears that prolactin is absolutely required, whereas LH is either permissive or facilitates CL function during pregnancy. Pre-implantation events are still poorly defined in the bitch, and no embryonic factors have been isolated or purified, preventing early pregnancy diagnosis. Parturition occurs following luteolysis, which results from the release of prostaglandin F(2alpha), which begins 36h prepartum in a process similar to that observed in other species. The role of estrogens at the time of parturition remains undefined.