As soon as a mare enters heat, her body experiences dramatic transformation. Uterus, ovaries and cervix become flaccid with thin walls and quiescent (Figure 1). Once estrogen becomes the predominant hormone and ovulation begins, reproductive organs change drastically; thicker and more vascular ovaries and an enlarged uterus become necessary to accommodate fertilized eggs which travel down through their respective oviducts to their place of implantation in the upper one-third of her uterus (Figures 2,3)
Once ovulation has occurred and the fertilized egg is placed in the upper one-third of the oviduct, implantation and placentation can begin in the lower two-thirds of the uterus. Implantation occurs via an oviduct that extends from an infundibulum–an area located at the ovarian end of the uterus–to carry both eggs and sperm to their site of fertilization.
Perineal conformation of mares is an integral factor in their ability to carry and deliver healthy foals. Mares with an asymmetrical rectum or sunken anus may be predisposed to rectal prolapse (Figure 3), which may lead to infections, obstruction of internal genitalia or abnormalities as well as uterine abnormalities.
Equine genitalia are safeguarded by a tough layer of fibrous tissue known as the perineum, located between anus and vulva. A normal conformation of perineum protects against entry of air or bacteria into genital tract, making it vulnerable to injury during parturition. Mares that experience rectal prolapse are more likely to endure difficult, prolonged, or traumatic birth experiences; routine veterinary care can reduce risks related to rectal prolapses.