What Is the Oestrous Cycle and Its Stages?

The oestrous cycle is the recurring reproductive cycle that most female mammals go through, driven by shifting hormones that prepare the body for potential pregnancy. Unlike the menstrual cycle in humans and some primates, the oestrous cycle does not end with the shedding of the uterine lining as a visible period. Instead, the lining is mostly reabsorbed. The cycle moves through four recognized stages, each with its own hormonal signature and physical signs, and the whole process can repeat every few days in a mouse or every few weeks in a cow. Understanding these stages matters well beyond biology classrooms, from livestock breeding programs to biomedical research design.

The Four Stages in Plain Terms

The names proestrus, estrus, metestrus, and diestrus date back to early twentieth-century research on guinea pigs, rats, and mice. In 1917, Stockard and Papanicolaou published the first thorough characterization of the changes occurring in the vaginal canal during the cycle, and their terminology became the standard that researchers still use today.

During proestrus, the ovaries are gearing up. Follicles grow and produce rising levels of estradiol, which thickens the uterine lining and shifts the types of cells visible in the vaginal canal. In cows, researchers have documented a progressive increase in both follicular size and estradiol production during the growth phase of each follicular wave.¹ This estradiol rise is what triggers the chain of events leading to ovulation. In species like mice, proestrus is brief, sometimes lasting less than a day.

Estrus is the stage of sexual receptivity, the window when a female will accept mating. It is the behavioral hallmark of the cycle and the origin of the word “heat.” Ovulation typically happens during or just after estrus, depending on the species. In mice, the hormonal pulse generator that controls gonadotropin-releasing hormone (GnRH) slows dramatically during estrus, dropping from roughly one synchronized burst every 40 minutes to about once every 10 hours.² That slowdown appears to be triggered by the progesterone released after ovulation.

Metestrus follows estrus and marks the transition into luteal dominance. The structure left behind after the egg is released, the corpus luteum, begins producing progesterone. This hormone quiets the uterus and creates conditions favorable for embryo implantation if fertilization has occurred. In cattle, strategies to boost progesterone during late metestrus and early diestrus, such as inducing an accessory corpus luteum using GnRH or human chorionic gonadotropin, are used to improve pregnancy outcomes in embryo transfer recipients.³

Diestrus is the longest stage in many species and the period when progesterone from the corpus luteum is at its peak. In cattle, serum progesterone concentrations during late diestrus (roughly days 9 through 17 of the cycle) average around 5.8 ng/mL, nearly double the levels seen in early diestrus.⁴ If no pregnancy takes hold, the corpus luteum eventually breaks down, progesterone drops, and the cycle starts over at proestrus.

How Scientists Identify Each Stage

In laboratory rodents, the go-to method is vaginal cytology. A small sample of cells is collected from the vaginal canal, placed on a slide, and examined under a microscope. The stages are defined by the absence, presence, or proportion of four basic cell types, along with how densely and in what arrangement the cells appear on the slide.⁵ Proestrus smears are dominated by nucleated epithelial cells. Estrus is characterized by large, flat cornified cells that have lost their nuclei. Metestrus shows a mix of cornified cells and white blood cells. Diestrus smears are loaded with white blood cells and a smaller number of nucleated epithelial cells.

A comparative study found that non-invasive vaginal lavage, where a small amount of saline is flushed into and out of the vaginal canal, can reliably identify stages based on the same three cell types: nucleated epithelial cells, cornified squamous cells, and leucocytes.⁶ In larger animals like cattle and horses, stage identification often relies on a combination of rectal ultrasound to track follicular and luteal structures, blood progesterone levels, and behavioral observation.

Spontaneous Versus Induced Ovulators

Not every mammal ovulates on a predictable schedule. Two fundamentally different systems have evolved. In spontaneous ovulators like rats, mice, sheep, and cows, the rising estradiol from maturing follicles triggers the brain to release a surge of luteinizing hormone (LH), and that surge causes ovulation. The cycle runs on its own internal clock, and mating is not required to release an egg.

Induced ovulators, including rabbits, ferrets, cats, and camels, take a different approach. In these species, the LH surge that triggers ovulation is not set off by hormones alone. Instead, it requires the physical stimulation of mating. Sensory signals from the genitals activate noradrenergic neurons in the brainstem, which then stimulate GnRH release from the hypothalamus.⁷ Without mating, ovulation simply does not happen. This distinction matters practically: a female cat that is not bred can cycle repeatedly without ovulating, sometimes staying in heat-like states for prolonged stretches.

Interestingly, the boundary between these two categories is not always clean. Some species considered induced ovulators will occasionally ovulate spontaneously under certain social or hormonal conditions, and some spontaneous ovulators can have their timing shifted by mating stimuli. The categories are better understood as ends of a spectrum than as a strict either-or.

The Hormonal Pulse Generator Behind It All

A group of specialized neurons in the brain’s arcuate nucleus, called kisspeptin neurons, act as the pulse generator for GnRH. These neurons fire in brief synchronized bursts lasting about 50 seconds, and each burst triggers a pulse of LH from the pituitary gland. In female mice, these bursts occur roughly once every 40 minutes throughout metestrus, diestrus, and proestrus, maintaining a remarkably stable rhythm.²

What surprised researchers is what happens during the LH surge itself. The pulse generator does not speed up or change its pattern at the onset of the proestrus surge. It keeps firing at the same frequency throughout the entire three- to four-hour surge, demonstrating that the pulse generator is not directly responsible for generating the surge.⁸ The surge appears to be driven by a separate mechanism, likely involving a different population of kisspeptin neurons in another brain region. The dramatic slowdown in pulse generator activity comes later, roughly four to five hours after the surge begins, and seems to be triggered by progesterone released after ovulation. Administering progesterone to mice during diestrus causes the same abrupt slowing, reinforcing the idea that post-ovulatory progesterone is the brake pedal.

How the Oestrous Cycle Differs from the Menstrual Cycle

People sometimes use “oestrous cycle” and “menstrual cycle” interchangeably, but they describe two different biological strategies. In both systems, a follicle matures, estradiol rises, ovulation occurs, and a corpus luteum produces progesterone. The key difference is what happens when pregnancy does not occur.

In species with a menstrual cycle, like humans and some other primates, the uterine lining builds up substantially and is shed as menstrual bleeding when progesterone drops at the end of the luteal phase. In species with an oestrous cycle, the uterine lining is mostly reabsorbed rather than shed. There is no external bleeding equivalent to a period, though some species may show minor vaginal discharge.

A second important distinction is behavioral. In most oestrous species, sexual receptivity is confined to estrus. Outside that window, the female actively rejects mating attempts. Human sexual behavior, by contrast, is not tightly linked to a specific cycle phase. Shared molecular machinery does exist between the two systems, though. The protein CRISP3, for instance, is found in the uterine lining of both mice (during the oestrous cycle) and humans (during the menstrual cycle), with increased presence during the proliferative phase in both species.⁹ The underlying reproductive machinery is conserved; how the cycle resolves each month is what diverged.

Pheromones and Social Influences

The oestrous cycle is not purely an internal affair. In rodents, chemical signals from other animals can speed it up, slow it down, or disrupt it entirely. Female guinea pigs exposed to the smell of concentrated male urine showed shorter oestrous cycles, with the shortening coming from a reduced period of vaginal closure. When researchers surgically removed the olfactory bulbs of female guinea pigs, the shortening effect disappeared, confirming that the signal was indeed a pheromone detected through smell.¹⁰

In mice, the picture gets more complicated. The genotype of both the male and the female matters. Researchers found that exposing one strain of female mice (strain E, which normally has very regular cycles) to males of a different strain (CN-) caused their cycles to become irregular, primarily by lengthening the diestrus phase.¹¹ The inhibitory effect was not simply a function of male testosterone levels; other pheromonal factors were also involved.¹² This means a female mouse’s responsiveness to male odors depends on her own genetic background, not just the intensity of the signal. These findings are well-established in rodents but have limited direct relevance to larger domestic animals, where behavioral and nutritional factors tend to exert more influence on cycle regularity.

Anestrus and What Stops the Cycle

Anestrus refers to a period when the oestrous cycle stops entirely. It is not a stage of the cycle itself but rather a state of reproductive dormancy. Several things can cause it.

Season: Many species are seasonal breeders. Sheep, deer, and horses cycle only during certain months of the year. The pineal gland’s secretion of melatonin, which changes with day length, conveys photoperiodic information that organizes these seasonal reproductive rhythms.¹³ Sheep, for example, are “short-day breeders” that begin cycling as days get shorter in autumn, while horses are “long-day breeders” that cycle in spring and summer.
Lactation: Nursing can suppress cycling. In mice, prolactin plays a central role in this lactational anestrus. Even after pups are weaned, the suppression can persist if prolactin levels remain elevated. Pharmacologically blocking prolactin with bromocriptine significantly shortened the time it took for mice to resume normal cycles after premature weaning and restored LH pulse frequency.¹⁴
Nutrition and stress: Poor body condition, malnutrition, and chronic stress are well-known suppressors of cycling in livestock. Animals in negative energy balance divert resources away from reproduction.
Age: As female mammals age, the oestrous cycle becomes irregular and eventually ceases. Aging is associated with disruption of the hormonal axis connecting the brain and ovaries, changes to the ovarian stroma, reduced egg quantity and quality, and altered uterine function that impairs both fertilization and embryo development.¹⁵

Heat Stress and Cycle Expression

In dairy cattle, high temperatures visibly suppress oestrous behavior. A study comparing summer and winter found that cows mounted each other five times more often in winter than in summer, and standing behavior, the classic sign of receptivity, was more than seven times more frequent in winter. The estrus detection rate fell from 86% in winter to just 53% in summer. Pregnancy rates showed an even more dramatic decline, dropping from 39% in winter to 10% in summer.¹⁶

Heat stress affects nearly every level of the reproductive axis. High body temperatures can impair follicle development, reduce the quality of the oocyte inside, and compromise the early embryo’s ability to survive. For dairy producers in warm climates, this is one of the single biggest obstacles to maintaining herd fertility, and it is getting worse as average temperatures rise. Cooling systems, timed artificial insemination protocols that do not rely on detecting behavioral estrus, and embryo transfer during cooler months are all strategies used to work around the problem.

Synchronization in Livestock Production

One of the most commercially significant applications of oestrous cycle biology is synchronization, the deliberate manipulation of a group of females so they come into estrus around the same time. This makes planned breeding, artificial insemination, and embryo transfer far more practical in large herds.

The development of synchronization protocols spans decades. Early work in the 1940s through 1960s used gonadotropins and progesterone, and by 1967, commercial progestogen products were available. Prostaglandin products gained approval in the 1970s and 1980s, followed by GnRH-based protocols.¹⁷ Modern protocols typically combine these tools. A common approach in cattle is to administer GnRH to reset follicular development, followed by a prostaglandin injection about a week later to cause the corpus luteum to regress, bringing the cow into estrus in a predictable timeframe.

These protocols do not work equally well across all cattle types. In crossbred cows with Bos indicus genetics, which are common in tropical and subtropical regions, researchers tested three synchronization-and-insemination protocols. Timed insemination protocols (CO-Synch and Hybrid-Synch) achieved pregnancy rates of about 31% and 36% respectively, significantly higher than the Select-Synch protocol where cows were inseminated only after showing natural estrus, which achieved just 21%.¹⁸ The takeaway for producers: in cattle that tend to show subtle or hard-to-detect estrus behavior, timed protocols that bypass the need for heat detection can meaningfully improve results.

The Oestrous Cycle in Biomedical Research

For decades, female rodents were underrepresented in laboratory research partly because of a concern that hormonal fluctuations across the oestrous cycle would add uncontrollable variability to experiments. This assumption has come under serious scrutiny. A large behavioral study comparing female mice in different oestrous stages found no significant difference in most behavioral measures between stages. Strain-to-strain behavioral differences were consistent regardless of cycle stage, suggesting the cycle has a minimal effect on detecting the kinds of behavioral differences researchers typically care about.¹⁹

A separate study went further, tracking individual female mice across many sessions and attempting to decode their oestrous stage from their spontaneous behavior. The result: each mouse had a characteristic behavioral signature that was far more identifiable than any pattern linked to cycle stage. Estrous state only negligibly impacted behavior, despite its known effects on neural circuits involved in movement and decision-making.²⁰ Subtle changes in behavior could be detected when data from multiple sessions in the same cycle phase were averaged together, but these were swamped by session-to-session variability in any individual experiment. These findings have provided strong empirical support for including female animals in behavioral research without requiring cycle-stage tracking in most contexts.

Cycle Length Across Species

One of the most striking things about the oestrous cycle is how wildly its duration varies. Mice and rats complete a cycle in four to five days. Dogs cycle roughly every six months, with an unusually long diestrus that lasts about two months regardless of whether the dog is pregnant. Cows cycle every 21 days on average. Horses cycle every 21 to 22 days during the breeding season but enter deep anestrus during winter. Pigs cycle every 18 to 24 days. Sheep have a cycle of about 17 days but only during the fall and early winter breeding season.

Even within a species, cycle length can shift. In cows, follicles develop in waves, with some cycles having two waves and others having three. The number of waves influences the total cycle length and the size of the ovulatory follicle.¹ Three-wave cycles tend to be a few days longer because of the extra time spent growing and regressing an additional dominant follicle before the final ovulatory wave. This variability within a single animal means that predicting exactly when a cow will come into heat requires more than just counting days from the last cycle.

Dogs are the real outliers. Their long interestrus interval and the fact that the corpus luteum persists for roughly the same duration whether or not they are pregnant make the canine cycle look more like a single-shot event than the tight loop seen in rodents. Some reproductive biologists have argued that the dog’s cycle is better described as a monoestrous pattern, cycling just once or twice a year, rather than the polyestrous pattern of species that cycle continuously throughout the year or throughout a breeding season.