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 are a precocial species, and foals are born capable of standing and running within a short time following birth .

Mares and foals.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 .

Depending on maturity, breed and the tasks expected, young horses are usually put under saddle and trained to be ridden between the ages of two and four . Although Thoroughbred race horses are put on the track at as young as two years old in some countries . horses specifically bred for sports such as dressage are generally not entered into top-level competition until a minimum age of four years old, because their bones and muscles are not solidly developed, nor is their advanced training complete . For endurance riding competition, horses may not compete until they are a full 60 calendar months (5 years) old .