Pigs do not have a “period” because they do not menstruate like humans or other primates. The term “period” refers to the menstrual cycle, which is characterized by the cyclic shedding of the uterine lining (endometrium) accompanied by visible external bleeding. Pigs, like the vast majority of non-primate mammals, follow the estrous cycle. This cycle prepares the female, known as a sow, for reproduction without the external discharge of blood and tissue associated with menstruation.
Why Pigs Do Not Menstruate
The fundamental biological difference between the menstrual and estrous cycles lies in the fate of the endometrial tissue. In the menstrual cycle, if pregnancy does not occur, the thick, blood-rich uterine lining that was prepared for an embryo is actively shed from the body. This shedding is what results in the visible bleeding.
In the estrous cycle, which pigs exhibit, the uterine lining is also built up in preparation for a potential pregnancy, but it is handled differently when fertilization fails. Instead of shedding this tissue externally, the sow’s body reabsorbs the endometrial lining. This physiological reabsorption prevents the occurrence of menstrual bleeding.
This mechanism of reabsorption allows the animal to conserve nutrients and blood components. The process of preparing and reabsorbing the uterine tissue is tightly regulated by hormones, ensuring a continuous reproductive system that does not involve external discharge.
The Mechanics of the Pig Estrous Cycle
The reproductive cycle of the domestic pig, or sow, typically spans 18 to 24 days, averaging 21 days. This cycle is controlled by hormone fluctuations that prepare the reproductive tract for potential conception. It is primarily divided into two main phases: the follicular phase and the luteal phase.
The follicular phase, which lasts about 5 to 7 days, is dominated by the hormone estrogen, secreted by developing ovarian follicles. Estrogen drives the preparation of the reproductive tract and culminates in the behavioral stage known as estrus, or heat, the period of sexual receptivity. Following estrus, the luteal phase begins, lasting approximately 13 to 15 days.
During the luteal phase, the ruptured follicles transform into structures called corpora lutea (CL), which secrete high levels of progesterone. Progesterone maintains the uterine lining and suppresses the development of new follicles, preparing the uterus for pregnancy. If the sow does not become pregnant, the uterus releases prostaglandin F2-alpha (PGF2α) around day 16, signaling the regression of the corpora lutea. This luteolysis causes a rapid drop in progesterone, triggering the reabsorption of the uterine lining and allowing the cycle to begin anew.
Physical Signs of Reproductive Activity
While pigs do not exhibit menstrual bleeding, they show distinct physical and behavioral changes when they enter estrus, the brief period of peak fertility. The most reliable sign that a sow is in heat is the “standing reflex,” or lordosis, where she stands rigid and motionless when pressure is applied to her back or when a boar is nearby. This behavior confirms receptivity.
Other physical indicators are related to the preparatory stages of the cycle, known as proestrus and estrus. The vulva often becomes visibly reddened and swollen, particularly in gilts (young female pigs). Increased vocalization, restlessness, and a reduced appetite are also common behavioral changes during this time.
A sow may display a clear or watery mucous discharge from the vulva, and rarely, minor bloody spotting may occur. This spotting is associated with the heightened vascularity of the estrus period, not the shedding of the uterine lining. These external signs indicate the sow is ovulating and receptive to mating.