From sexual maturity onwards (approx. 10 to 24 months of age), mares undergo a regular seasonal physiological cycle that combined with their anatomy, facilitates the opportunity for conception, gestation and the birth of offspring. 
(through understanding of reproductive anatomy, physiology and endocrinology essential to maximisation of reproductive efficiency and fertility rates)

Anatomy 

Situated inside the pelvic and abdominal regions, the mare has an extensive reproductive tract that in its most basic form, can be considered as a Y- shaped tubular organ with a series of constrictions along is length. (figure). Each constriction has a specific function integral to the reproductive success/viability of the mare.

Vulva – the external opening of the mare’s reproductive tract which protects the entrance to the vagina. The vulva is also the location of the clitoris and clitoral sinuses; important in oestrus behaviour and disease prevention respectively (see relevant pages) 

The Perineum – the outer vulva, anus and surrounding area. The confirmation of this area is of significance due to its function in protection of the reproductive tract from contamination with air, solids and bacteria (see relevant pages)

Vagina - part of the birth canal that lies between the vulva and the cervix. On average 18-23cm in length and 10-15cm in diameter, the vagina produces secretions which act as a second protector for the uterus from bacterial contamination. 

Cervix – a highly muscular structure approximately 10 cm in length that lies between the vagina and the uterus. It marks the opening of the uterus and serves to maintain a sterile environment. It relaxes when the mare is ‘in season’ and closes when not receptive to the stallion and during pregnancy.

Uterus – a hollow, muscular organ joining the cervix and fallopian tubes. The uterus sits in the abdominal cavity and is held in place by two ligaments on either side of the vertebral column. Consisting of two areas, the body and the horns, the uterus is the site of embryonic development and nourishment. It also produces hormones and is the location of semen deposition during natural breeding.

Utero-tubular Junction – a muscular constriction between the uterus and the fallopian tubes (see below). The utero-tubular junction has a unique function in that it only allows fertilised egg cells to pass from the fallopian tubes into the uterus.
Fallopian tubes/ oviducts – the mare has two fallopian tubes/oviducts, which extend from the end of the uterine horns to the infundibulum (see below). They provide the site of fertilisation of the sperm and egg cell and transport the fertilised egg cell to the uterus.  
Infundibulum (fimbria) - a "catcher's mitt" structure at the ovarian end of the fallopian tubes that ‘catch’ the egg cell released from the ovary at ovulation and transports it down into the oviduct.

Ovary - the primary sex organ of the mare. Mares have two ovaries which are bean shaped structures found at the end of the oviduct. The ovaries produce the egg cell to be fertilized by the male sperm cell whilst also serving as an endocrine gland, producing the hormones oestrogen and progesterone.
Broad ligament - a tough layer of fibrous tissue, containing blood vessels and nerves that suspends the reproductive tract in the abdomen.

Physiology 

Mares are seasonal breeders with the natural period of breeding spanning from March/April to September i.e. the spring/summer months. Increasing day length and environmental temperature are the main stimulants for the start of the mares’ oestrus cycle and receptivity to breeding. 
The first oestrus phases of each year are often erratic and prolonged, during which time mares may be in season for 20-30 days or more. During late March/ April, most exhibit sexual receptivity, and from late April through to August, most mares will cycle normally.
Oestrous Cycle
The oestrous cycle is composed of two phases: the oestrous phase (in heat/ in season phase) and the dioestrus phase (out of heat, non-receptive to the stallion). Each cycle lasts for 21 days (±3 days). Oestrus usually lasts for 6 days, but can range from 4-10 days, depending on the mare. Dioestrus normally lasts for 15 days but can vary from 12-18 days. From September through to March, mares will naturally go into a period of anoestrus, where little to no reproductive cycling occurs and conception is unlikely.
Ovulation, the release of the egg from the ovary, can occur at any time during the oestrus phase. However, it normally takes place 24-48 hr before the end of the oestrus period. To maximize the chance of conception, breeding should occur within 12 hrs of ovulation.
Endocrinological Control
The oestrous cycle is controlled and driven by the interaction of hormones; chemical substances created by the body that control many bodily functions. The main ones involved with the cycle are follicle stimulating hormone (FSH), oestrogen, luteinising hormone (LH), progesterone and prostaglandin. As summarised in figure x, these hormones drive the development and maturation of ovarian follicles, leading to ovulation whilst timely stimulating the mares’ receptiveness for breeding. 

The cycle begins with the increased production of follicle stimulating hormone (FSH) by the pituitary gland. As spring approaches, the pituitary gland located at the base of the brain is stimulated by increased day length to increase its production of FSH. The hormone is released into the blood and acts on the ovaries to initiate the development of a follicle containing an egg cell/ovum. The developing follicle produces oestrogens, which are also released into the blood.
Oestrogens have several functions. When blood oestrogen reaches a certain level, its stimulates a surge of luteinizing hormone to be released from the pituitary gland. Oestrogens are also responsible for the behavioural signs of oestrus (‘in season’ behaviours) whilst also acting on the oviducts, uterus and cervix to prepare the reproductive tract for pregnancy.
The surge of luteinizing hormone causes the follicle on the ovary to rupture, resulting in ovulation. As the follicle develops on the ovary, the ovum (egg) inside the follicle undergoes several changes to become capable of fertilisation by a sperm cell. The follicle also contains a viscous fluid and when its  wall ruptures, the fluid flows out, carrying the ovum with it. The cavity left by the ruptured follicle becomes engorged with blood and forms a structure known as a corpus hemorrhagicum. The corpus hemorrhagicum then luteinizes to form the corpus luteum, also known as the yellow body.
As the corpus luteum develops, it starts to produce progesterone, which acts on the pituitary gland and reproductive tract. The feedback of progesterone via the blood inhibits the release of luteinizing hormone from the pituitary galnd. Under the influence of progesterone, the mare no longer shows oestrus behaviours and is non receptive to breeding. Progesterone function is to maintain the pregnancy by maintaining a uterine environment conducive to foetal development.
If conception does not occur, the corpus luteum remains functional for 12-14 days. At this time, prostaglandin is released from the inner lining of the uterus. Prostaglandin has a luteolytic effect - it acts on the corpus luteum and causes it to degenerate. As the corpus luteum regresses, progesterone levels are reduced, resulting in the removal of the inhibition to luteinizing hormone secretion. In this way, the oestrus cycle is permitted to start again.
If the mare conceives, hormonal activities are essentially the same as for the 12-14 days post-ovulation. Pregnancy recognition is stimulated by the action of the developing embryo migrating throughout the uterus; this action inhibits prostaglandin release. The result is an anti- luteolytic effect, so the corpus luteum remains functional, progesterone levels are maintained and the pregnancy is continued.