Signs of impending birth in a pregnant mare include both physical and behavioral changes. If your mare has foaled before, it's likely she will carry subsequent foals for approximately the same time. That's important information on calculating her due date, if you happen to have it. While the average equine gestation period is 340 days, it's perfectly normal for a birth to proceed a week or so earlier or later. Most mares foal late at night, and seem to have an uncanny ability to wait until a person on foal watch falls asleep or takes a break. If your mare is at home, a video monitoring system or similar equipment allows you to observe your horse from the privacy of your home and rush to her when it's clear she's giving birth.
Certain behavioral changes indicate that baby will arrive soon. Some early signs of impending delivery resemble those of colic. In the initial stages of labor, she'll likely become restless. If outdoors in a herd, she'll isolate herself from other horses. She'll stop eating. Lying down and quickly rising again becomes a pattern. The mare urinates frequently, passes small quantities of manure and may break out into a sweat. Her tail swishes back and forth. This first stage ends when her water breaks, with several gallons of fluid are discharged from her vagina.
Your mare's udder starts changing approximately a month before the foal is due. Although the udder is enlarged during this period, as her due date approaches her teats change direction, pointing somewhat sideways rather than down. Some -- but not all -- mares will start waxing within 36 hours of giving birth. Small amounts of colostrum will appear on the teats. If this is a mare's first foal, mammary changes may be less evident.
Within a day or so of foaling, you'll see the mare's vulva and tailhead relax. However, it's not as accurate a predictor in mares who have had many foals -- their vulvas might relax well before giving birth -- or in mares experiencing their first foaling. It's harder to establish whether tailhead relaxation has occurred in obese mares.
A horse's normal body temperature is approximately 100 degrees. If you take your mare's temperature several times daily, you should notice a dip in temperature about four hours before foaling starts. It's not the most reliable indicator, as not all mares exhibit the drop in temperature and many mares become stressed by frequent rectal temperature-taking.