ABSTRACT
Pregnancy may occur during metestrus or diestrus as a result of a fertile mating. Gestation length varies with species, from 31 days in the Western chipmunk to 660 days in the African elephant.
Proestrus is the period after the CL fails (usually due to PGF2), when progesterone levels drop, FSH release stimulates follicular growth, and rising estrogen levels lead to estrus. Proestrus and early estrus are referred to as the follicular phase (before ovulation). Proestrus is short (two to three days) in domestic animals compared to the follicular phase of menstruating primates (14 days), due largely to the fact that regeneration of the endometrial stratum functionale is unnecessary in domestic animals, because it does not fully degenerate when the CL fails (Ch. 56). Pseudopregnancy, or false pregnancy, is an exaggerated diestrual response of the bitch and queen. It may be related to the extreme sensitivity of the canine endometrium and mammary glands to progesterone, in synergy with prolactin. The CL of the nonpregnant bitch remains functional for an extended period of time after ovulation. In overt pseudopregnancy (which may sometimes last as long as pregnancy), mammary glands develop, the uterus enlarges, the abdomen may relax, the pelvis and external genitalia may change as they would during pregnancy, and the bitch may develop a whelping nest. In fact, some pseudopregnant bitches have been reported to adopt and effectively nurse puppies from other bitches. Queens, rodents, and rabbits that are induced to ovulate by mechanical stimulation of the vagina, exogenous hormones, or matings with sterile males may also become pseudopregnant. Pseudopregnancy lasts from 30 to 70 days in the queen, yet it is not associated with the profound organic and behavioral changes seen in the bitch, and seldom leads to lactation and nesting behavior. However, pseudopregnant queens undergo vaginal, uterine, and oviductal changes induced by progesterone secreted by CLs.
