Gestation of the Mare, Explained
The gestation period of the mare is less well-defined as compared to many other mammals. In other words, the window for parturition in the mare is much broader than that of other mammals. Mares are seasonally polyestrus, which means that their breeding season is very well-defined (even if their delivery window isn’t!), with multiple heat (estrus) cycles, and a non-breeding season (anestrus) during Winter.
The gestational period of the mare is between 320 and 370 days, this being considered within normal limits. Some extend this range even further, with a 380-day gestation being within the normal range.1 Many studies have been done evaluating the gestation lengths in mares. One of these examined the gestation lengths of Standardbred mares at two commercial farms over a two-year period. Of the 594 foals resulting from these records, the mean gestation length was 349 days. Further examination of data revealed a mean of 347 days for fillies and 350 days for colts.2
Gestation and Foaling
Foals are considered premature if delivered at less than 326 days of gestation. In such cases, the foal will usually require immediate veterinary attention. Mares foaling before 310 days are considered to have aborted.3 Many breeders will calculate the expected foaling date at 11 months following the last breeding date. Foals born between 300 and 320 days frequently require neonatal care. Foals born after 370 days of gestation usually do not present problems, though some tend to be of lower birth weight than expected. This is thought to be due to delayed uterine development.
Various traits can influence the length of a mare’s pregnancy. Most of the literature holds that colts tend to gestate between two to seven days longer than fillies. The nutritional regimen of the mare can also influence the length of her gestation, with thinner mares having a slightly longer gestation length than mares in a more optimal body condition.1,3 Mares foaling during the late Spring and Summer usually have a shorter gestation length than those foaling in January or February. Exposing bred mares to artificially lengthened days during the latter third of gestation has been shown to shorten gestational length by about one week.
Parturition
Most mares will have a normal parturition without any assistance. However, given the investment of money and time that horse owners place in their broodmares and foals, many are reluctant to take that risk. Thus, owners and breeders often have their own time revolve around the mare as she nears parturition. In order to reliably predict when the mare will give birth, several commercial kits to estimate foaling time have been developed. These kits use the standard magnesium concentrations in the pre-partum mammary secretions of the mare to determine when parturition will occur. Most mares show a significant rise in calcium and magnesium concentrations just prior to parturition, which can allow attendants using such kits to estimate time of foaling to within 12 to 24 hours.2,3 The kits are available from veterinarians and veterinary supply companies, and are reasonably-priced and easy to use.
Parturition is broken down into three stages. In Stage 1, the mare’s body is preparing for delivery, although many of the signs are not visible externally. This first stage usually lasts from one to four hours, during which time the initial uterine contractions and final positioning of the foal for delivery take place. The mare is likely to appear skittish and uncomfortable, and may exhibit:
- Restlessness
- Pawing the bedding or ground
- Tail switching
- Getting up and down frequently
- Sweating in the flanks
- Frequent urination
Stage 2 is considered to be the most critical stage of parturition, since this is when the foal emerges. If foaling is proceeding normally, the mare should be left on her own. This second stage, what would be called “hard labor” in a human, is usually completed in 10 to 30 minutes. The mare should be observed closely, but there should be no intercession unless absolutely necessary.
In this stage, the mare will experience heavy contractions and will lie on her side. The water bag will appear, and should break on its own. While dystocia (foaling difficulty) only occurs in around 10% of mares, it is important to ensure that the foaling attendant is familiar with both normal and abnormal foal presentations (this will of course be unnecessary with a veterinarian present).3 In normal presentations, the foal’s two front feet will appear first, with one foot slightly ahead of the other. The nose and head should appear next, with the back of the foal toward the mare’s back. If the presentation diverges from this, the veterinarian should be called immediately, as the foal could suffocate.
In cases where the foal is delivered except for the hips and rear legs, gently pulling in a downward direction toward the mare’s heels will help her complete the delivery. Bear in mind that not all abnormal presentations are life-threatening; many can be easily remedied by a seasoned attendant. Horse owners who will be on their own during foaling (without a veterinarian or seasoned attendant) should consult with the veterinarian prior to foaling to learn how to handle abnormal foal presentations.
When the foal is almost completely delivered, the mare may rest, allowing the foal's hind legs to remain in the birth canal for a period of time before they are pushed out. Once the foal has emerged completely from the birth canal, it is important to gauge the mare’s condition and mood from a distance. If the mare is recumbent, standing too quickly may prematurely rupture the umbilical cord, so it is critical not to incite her to stand by entering the stall right away. Stage 2 is completed when the umbilical cord is broken as the mare stands up.
Stage 3 is the final stage of parturition, which encompasses the delivery of the placenta. These membranes which surround the foal during gestation should be expelled three to four hours after delivery. Once expelled, the placenta should be examined to determine if it is intact or if any portions may have been detached. Placenta retention can be a problem, leading to uterine infection (endometritis) and/or laminitis (founder).3 The placenta should not be forcibly pulled from the mare under any circumstances, as this could tear or prolapse the uterus or result in contamination of the uterus with pieces of torn placenta.
In most cases, the mare will stand with the placenta partially expelled while the foal learns—very quickly—to stand and nurse. As with most mammals, the foal’s nursing stimulates uterine contractions, which aids in the expulsion of the placenta.
1Espy, B. Equine Reproduction from conception to birth. American Association of Equine Practitioners (Accessed 5 March 2020.).
2Dicken, M., E.K. Gee, C.W. Rogers, and I.G. Mayhew. 2012. Gestation length and occurrence of daytime foaling of Standardbred mares on two stud farms in New Zealand. New Zealand Veterinary Journal 60:42-46.
3Anderson, K. The Foaling Mare. 2008. University of Nebraska Extension Publication G1874.
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