The "average" horse gestation period is between 320 and 362 days. Most foaling calculators use the middle dates (340 -343) as their calculation from the last breeding date. It is not unusual for your mare to develop a predictable pattern - for example, our Blue Angel had a history of foaling earlier than that 340-343 day average - she carried Mudcat 323 days, Nellie 324 days and Hemingway 339. Once you know your mare's history it's much easier to determine when she will foal - but with an unknown or maiden mare it's a little harder to predict!
It's also helpful to know & watch for the signs your mare will give you as she nears the big day! Keep in mind that not every mare will give every possible sign - some of them like to keep you guessing. In the final few weeks and days before she foals, most mares will show some of the following signs:
She will begin to "bag up" as her milk comes in and her udders will increase in size and become firm and warm. This usually happens in the last month.
The foal will "drop" - the sides of the mares belly will go from looking very round to more flat and the bottom of her belly will hang lower and may even begin to look a little pointed.
Liquid can be expressed from her teats - at first a golden color, then becoming paler as birth becomes immanent.
The muscles around her tail head will relax. This may cause her to look a little hollow around the base of her tail - or if you poke in this area it will feel soft & a little jiggly as if she has pudding under her skin.
Her vulva will relax and begin to elongate.
She will develop small drops of wax on the ends of her teats. About 75% of horses will wax up - and this is usually a good sign that she will foal within the next 48 hours (keep in mind, not every mare will cooperate with the rules of prediction!). If the wax drops off and she begins to drip milk - it's a good chance that you will have your new baby within the next 24 hours! (Blue Angel delivered Mudcat about 30 hours after we noticed the wax, we saw wax one evening, the next evening she was dripping and then she foaled the following morning at 7 a.m. She was a little faster with Nellie - she had wax in the evening and foaled about 1:30 a.m. that same night. With Hemingway she had wax in the morning and foaled at 3 a.m.)
She may also show behavioral changes - seeming restless or moody, she may lay down & get up more and may have less of an appetite during the final days and hours.
There are various test kits to test the mare's milk to determine when she will be ready to foal and alarm devices to let you know when labor has started. Some people install cameras or baby monitors to help when they are on "foal watch". Whatever tools you use to predict the birth - it's certainly a blessing to see a new baby come into the world!
A note on fescue: Feeding fescue hay or grass during the last few months of pregnancy can potentially cause problems with the pregnancy and/or with the milk production. This is caused by an endophyte that grows on some fescue grasses. We take our mares off pasture during the last trimester and begin feeding them fescue-free hay to avoid these complications. :)