Progesterone is a female hormone (common in most mammals), of which the production is unique during the estrous (heat) cycle of female dogs. The early rise of progesterone during estrus is from the luteinized follicles (A yellow mass of cells that forms from an ovarian follicle during the luteal phase of the menstrual cycle in mammals; it secretes steroid hormones. ) of the ovaries and can be used in breeding management.
The female dog (also known as a bitch or dam) ovulates ~2–3 days after the LH peak. After ovulation, the follicles are replaced by corpora lutea (CLs), which produce progesterone.
The production of progesterone continues throughout diestrus, the phase of the estrous cycle after estrus, regardless of whether the bitch is pregnant. The duration of diestrus is the same as that of pregnancy, ~62–63 days from the LH peak. Unlike in other domestic species, in the bitch there is no production of prostaglandin F2α from the endometrium to cause luteolysis (CL regression). During diestrus, many bitches go through pseudopregnancy in which they may gain weight and have an enlarged abdomen; other overt signs may be seen, including mammary gland enlargement, as well as prewhelping behavior such as nesting or “adopting” toys and shoes. The pseudopregnancy (or diestrus) is considered a normal occurrence and may not need any treatment.