Do Horses Have Periods? Myth or Fact.

Horses are majestic animals that have always been a favourite among people. These animals are known for their grace, beauty and strength, and they have been used for transportation, agriculture and sports for centuries. However, one of the lesser-known aspects of a horse’s biology is their reproductive cycle. Many people wonder if horses have periods, and if so, how they differ from human menstruation. In this article, we will explore the myth and fact surrounding the question of whether horses have periods.

What are Periods?

Before we can explore whether horses have periods, we need to understand what human menstruation is all about. Menstruation is a natural process that occurs in females of reproductive age. It is a monthly shedding of the uterine lining that occurs in response to hormonal changes in the body. When the uterus sheds its lining, it causes bleeding from the vagina, which can last from three to seven days.

Do Horses Have Periods?

The simple answer is no. Horses do not have periods like humans do. While they do have a reproductive cycle, it differs significantly from the menstrual cycle of humans. The reproductive cycle of a female horse is called the estrous cycle or heat cycle. It is composed of recurring periods of sexual receptivity and fertility, followed by periods of infertility.

What is the Estrous Cycle?

The estrous cycle is the reproductive cycle of female horses. It is also referred to as the heat cycle because mares are only sexually receptive during this time. The estrous cycle lasts for around 21 days in most horses, and it is divided into four distinct phases: proestrus, estrus, diestrus, and anestrus. Unlike human menstruation, the estrous cycle does not involve periods of bleeding from the vagina.

  • Proestrus: This is the first phase of the estrous cycle, and it lasts for about four days. During this phase, the mare’s body is preparing for ovulation, and the follicles in the ovary start to grow.
  • Estrus: The estrus phase lasts for around three to seven days, during which time the mare is sexually receptive. Her behaviour changes, and she may become more vocal and restless. She may also urinate frequently to attract males. This is the time when mares are most likely to conceive.
  • Diestrus: This is the period after estrus, and it lasts for about 14 to 15 days. During diestrus, the mare’s body prepares for pregnancy. If fertilisation does not occur, the mare will return to proestrus, and the cycle will begin again.
  • Anestrus: This is the phase during which the mare is not sexually receptive or fertile. It lasts for around 4 to 6 months.

Menstruation vs Estrous Cycle

The menstrual cycle and the estrous cycle differ in many ways. Firstly, the menstrual cycle is a monthly occurrence in human females, whereas the estrous cycle is seasonal in horses. Secondly, humans experience periodic bleeding that lasts for 3 to 7 days, whereas mares do not have bleeding during the estrous cycle. Finally, during the estrous cycle, mares are only sexually receptive for a few days, whereas humans are sexually receptive throughout their menstrual cycles.

Why is the Estrous Cycle Important?

The estrous cycle is important because it is the primary way that horses reproduce. Breeders use knowledge of this cycle to determine the optimal time for breeding. It can also help identify fertility issues if a horse is not getting pregnant or if they are experiencing difficulties during pregnancy.

What Happens if a Mare Does Not Get Pregnant?

If a mare does not get pregnant during the estrous cycle, she will return to proestrus, and the cycle will start again. This process will continue until the mare does conceive. However, there are some instances where a mare may not be able to conceive due to age, injury or other factors.

In such cases, the mare will go through the entire estrous cycle but will not become pregnant. This is known as a silent or anovulatory cycle. If the mare continues to have anovulatory cycles, it may indicate infertility or other reproductive health issues.


While many people may wonder if horses have periods, the answer is no. Horses do not experience menstrual cycles like humans. Instead, they go through the estrous cycle, which is a series of repeating periods of fertility and infertility. Understanding the estrous cycle is important for horse breeders and owners, as it can help identify potential fertility issues and optimise breeding time.

Frequently Asked Questions

  • Do horses have periods every month?No, horses do not have periods every month. They have a seasonal reproductive cycle called the estrous cycle.
  • Do horses bleed during their estrous cycles?No, horses do not bleed during their estrous cycles. While some mares may have a slight discharge, it is not the same as menstruation in humans.
  • How often do horses go through their estrous cycles?Horses go through their estrous cycle every 21 days, on average. However, there can be variation between individual mares.
  • Can horses get pregnant during their estrous cycles?Yes, horses can get pregnant during their estrous cycles. The estrous cycle is the prime time for breeding, and breeders use it to optimise the chances of conception.
  • What causes a mare to go into estrus?The hormonal changes that occur during the estrous cycle cause mares to go into estrus. Specifically, increased levels of estrogen trigger behavioural and physical changes that signal sexual receptivity.


  • McDonnell, S. et al. (2011). Reproductive Anatomy and Physiology of the Mare. Veterinary Clinics of North America: Equine Practice, vol. 27, no. 1, pp. 1–20.
  • Ginther, O.J. (2016). Reproductive Biology of the Mare: Basic and Applied Aspects (2nd ed.). Wiley-Blackwell.
  • Hart, K.A. (2012). Clinical Aspects of Reproductive Physiology in Horses. Theriogenology, vol. 78, no. 9, pp. 1821–1828.

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