Pregnancy lasts for approximately 335-340 days and usually results in one foal. Twins are rare. Colts are carried on average about 4 days longer than fillies..
Horses, particularly colts, may sometimes be physically capable of reproduction at approximately 18 months but in practice are rarely allowed to breed until a minimum age of 3 years, especially females. Horses four years old are considered mature, though the skeleton usually finishes developing at the age of six, and the precise time of completion of development also depends on the horse's size (therefore a connection to breed exists), gender, and the quality of care provided by its owner. Also, if the horse is larger, its bones are larger; therefore, not only do the bones take longer to actually form bone tissue (bones are made of cartilage in earlier stages of bone formation), but the epiphyseal plates (plates that fuse a bone into one piece by connecting the bone shaft to the bone ends) are also larger and take longer to convert from cartilage to bone as well. These plates convert after the other parts of the bones do but are crucial to development.
Your welcome I'm a total newbie here on the forum and am just researching and getting my fencing up this summer so I can buy a horse next spring. I never new horses has such a long gestation period. I hope everything goes well for your mare.