Gestation in the mare is a lengthy period of development that requires careful management. Unlike many other domestic species, the duration of a mare’s pregnancy is highly variable, making the precise prediction of the foaling date a challenge for owners and veterinarians. Understanding the approximate timeline is paramount for providing appropriate nutritional support, preparing the foaling environment, and ensuring the health of both the mare and her developing foal.
The Average Gestation Period for Mares
The term “gestation” refers to the period from conception to birth, and in horses, this timeline is characterized by a significant range of normal variation. The most commonly accepted average length of pregnancy for a mare is approximately 340 days, which is roughly 11 months.
The standard range considered healthy and viable for a mare’s gestation is between 320 and 365 days. Foals born before 320 days are often considered premature and may require intensive veterinary care to survive. While 340 days serves as a useful benchmark, a delivery anywhere within the 320-to-365-day window is considered a normal biological outcome.
Key Factors That Influence Pregnancy Duration
The wide variation in gestation length is influenced by a combination of biological, environmental, and fetal factors. Genetic differences play a role, as certain breeds have slightly different averages; for instance, some draft horses may carry longer, while ponies tend to have a shorter gestation period. Individual mares also tend to repeat their own unique gestation length in subsequent pregnancies, making past history a valuable prediction tool.
The sex of the developing foal can affect the duration of the pregnancy. Mares carrying a male foal (colt) often have a gestation that is a few days longer than those carrying a female foal (filly). This difference is hypothesized to be linked to the influence of testosterone or sex chromosome-linked effects.
Environmental factors, particularly the time of year when the mare is due to foal, have a notable impact on the length of pregnancy. Mares scheduled to give birth earlier in the year, such as late winter or early spring, often have gestations that are five to ten days longer than those foaling in late spring or summer. This phenomenon is primarily related to the mare’s response to the increasing length of daylight, or photoperiod. The mare’s age and whether she is a first-time mother (maiden mare) can also lead to minor variations.
Confirming and Monitoring the Mare’s Pregnancy
Confirmation and consistent monitoring of the pregnancy are necessary for proper management. Transrectal ultrasonography is the preferred and most accurate method for early diagnosis, allowing veterinarians to confirm pregnancy as early as 14 to 16 days after breeding. This early scan is especially important for checking for the presence of twins, which are a major health risk in horses, and confirming a viable heartbeat around 25 to 30 days.
Transrectal palpation, where a veterinarian manually assesses the uterus, is another long-standing method used to check for uterine tone and the size of the developing fetus throughout the pregnancy. As the pregnancy progresses, hormone testing provides functional information about the health of the feto-placental unit. For example, measuring Pregnant Mare Serum Gonadotropin (PMSG), also known as equine Chorionic Gonadotropin (eCG), is a common test performed between 40 and 100 days post-breeding.
Signs That Foaling is Imminent
As the mare approaches her estimated due date, several distinct physical and behavioral signs signal that birth is imminent, typically within 24 to 72 hours.
Physical Indicators
One of the earliest and most noticeable physical signs is the enlargement of the udder, often called “bagging up,” which usually begins two to six weeks prior to delivery. The teats will engorge with colostrum, and the fluid secretion will change from a clear liquid to a thick, honey-like consistency as the foaling day nears.
A strong indicator is “waxing,” the accumulation of small, waxy, yellowish beads of dried colostrum on the ends of the mare’s teats. While this sign is present in about 70% of mares and often precedes foaling by 24 to 48 hours, it is not always a precise predictor. The mare’s body also prepares for birth by relaxing the pelvic ligaments, which causes a noticeable softening and sinking appearance around the tail head.
Behavioral Changes
Behavioral changes also become apparent in the final hours leading up to delivery. The mare may become restless, frequently lying down and standing up, or exhibit signs similar to mild colic by looking at her flanks. Many mares will instinctively seek a quiet, isolated spot to deliver their foal, a behavior rooted in their nature as prey animals.