Kathy Anderson, Extension Horse Specialst, University of Nebraska
The average gestation length (duration of pregnancy) of mares is 335 to 340 days, but can range from 320 to 370 days. There may be much variability among mares, but most individually follow similar patterns year after year. Foals are considered premature if delivered at less than 326 days of gestation and usually require immediate veterinary attention. Mares foaling before 310 days are considered to have aborted. Many breeders will calculate the expected foaling date at 11 months following the last breeding date. Most mares will gestate longer than 11 months, however, this allows for fewer "surprise" foalings in undesirable conditions.
There are various traits which can influence length of a mare's pregnancy. Studies have shown colts tend to gestate 2 to 7 days longer than fillies. The nutritional plane of a mare also has been shown to have some influence on gestation length. Additionally, mares foaling during the late spring and summer months (long days) tend to have a shorter gestation length than those foaling in January or February. Exposure of bred mares to artificially lengthened days during the latter third of gestation has been shown to shorten gestational length by about 10 days.
The signs of impending parturition are about as variable as gestation length. Although there is a tremendous amount of variation among mares, many individuals do repeat their foaling behavior year after year. Therefore, it is recommended to keep accurate records on each mare to aid future deliveries.
The classical signs of approaching parturition include udder development beginning 2 to 6 weeks before foaling, then milk let down into the teats (teat distension) 4-6 days prior to parturition. Due to oozing out of some of the colostrum, the ends of the teats become covered over and the mare is said to be "waxing", which occurs 1-2 days before foaling. At this time, some mares will have occasional dripping or streaming of colostrum. It this streaming is continuous, the colostrum should be collected and stored for possible use with the newborn foal. In addition to changes associated with the mammary gland, muscular relaxation in the pelvic region occurs progressively during the last 7-14 days of gestation and the mare's vulva begins to relax during the final days of gestation.
Many mares will show these "classical" signs of approaching parturition and make it fairly easy to determine their foaling time. On the other hand some mares will break all the rules and these signs may not be present, or may appear at varied times.
Early in gestation, some mares require a Caslicks operation to partially suture together the lips of the vulva. Caslicks are used to prevent problems in mares that have abnormal vulva conformation. Mares that have a Caslicks must have the lips of the vulva opened at least 30 days prior to foaling. If the Caslicks is not opened, there is a possibility of oblique tears to the vulva or vagina which are difficult to repair and may result in a deformity that leads to uterine infection.
A foaling mare should not be placed in a strange environment or have a stranger act as night attendant immediately prior to parturition. Any sudden changes may delay foaling. Ideally, mares should be placed in the foaling environment 2 to 3 weeks prior to her expected foaling date.
Mares can foal in a variety of locations, depending on the weather and facilities available. Whatever the choice of foaling locations, the environment should be clean, have adequate space, and be reasonably quiet. Mares due to foal in the winter months will require a large (14' X 14' minimum), clean foaling stall. During warm weather, many producers choose to allow their mares to foal in grassy paddocks or pastures. Dirt lots should be avoided if possible.
For mares foaling in a stall, the stall should be freshly bedded with clean, dry straw rather than shavings. An 8 to 10 inch-thick bed of straw will decrease duts, chances of infection, and is easier to clean. Safety to the mare and foal should be kept in mind when selecting a foaling stall. Stalls should be constructed to allow isolation of the mare and safety to the newborn foal. Thorough disinfection of the stall prior to bringing in the mare will help prevent disease. The mare should be allowed ample exercise up to foaling. Stall confinement for an extended period just prior to foaling may predispose the mare to impaction colic and abnormal swelling.
Mares foaling in paddocks or pasture should either be isolated or have sufficient space to separate themselves from any other horses in the pasture. Additionally, the pasture/paddock should be examined for possible hazards to the foal. A shelter should be provided in case of wet or cold weather.
Regardless of the place, the foaling area should be isolated and quiet. Safety of the mare and foal should be kept in mind when deciding where the foaling will occur. The cleanliness of the foaling area cannot be stressed enough. Foaling in contaminated areas can predispose the foal to bacteria invasion and neonatal septicemia via the navel stump.