Establishment and Maintenance of Feline Pregnancy-A Comprehensive Review

Cats are different from dogs, and many questions remain open concerning the establishment of pregnancy. In cats, as in dogs, no feto-maternal signaling leading to establishment of pregnancy is known. But as opposed to dogs, the placenta is a source of steroid hormones and corticotropin-releasing hormone (CRH). Scarce information is available on physiological mechanisms at the uterine level during early gestation; more studies are needed on lymphocyte subsets, feto-maternal crosstalk and other mechanisms leading to local immunosuppression, allograft acceptance and embryo nidation and invasion. Recent studies investigate the function of extracellular vesicles (EVs); however, there is no study on embryo- or endometrium-derived EV. During pregnancy, anti-Müllerian hormone (AMH) serum concentrations were found to be higher than in non-pregnant cats, and a recent study found that supraphysiological levels may lead to pregnancy loss; the function of AMH during pregnancy warrants investigation. Most information is available on corpus luteum development and function, showing some similarities to dogs. Some information on maintenance of feline pregnancy was obtained by ovariectomy (OE) or the use of endocrine disruptors, showing that OE does not lead to pregnancy loss in all cases, especially when performed after day 35; the variable effect is still not fully understood. Antiprogesterone, dopamine agonists and prostaglandins were used in different dosages and treatment schemes and showed variable effect during the second half of gestation, highlighting progesterone and prolactin as key hormones for the maintenance of gestation. Some events during early gestation are comparable with the canine species, even though they appear earlier, like the entrance of the zygote into the uterus and implantation; however, significant differences are present concerning the histomorphology of the placenta and, in a few cases, even the gross morphology as in some cats, where the zonary placenta does not completely surround the fetus. Sonographical monitoring of feline pregnancy requires knowledge of species-specific developmental steps and the differential appearance of fetal and maternal structures in comparison with dogs.