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Basic Gestation Facts

Newly born foal
Foals can stand within a very short time after birth.  
Horses are mammals, and like all mammals give birth to live offspring who are nourished for the first part of their life by their mother's milk. A mare can only produce one foal per year. A stallion is capable of producing around two hundred foals per year. A mare is capable of producing a foal from about eighteen months of age, but it is healthier for mare and foal if the mare is at least four years old. A stallion can start breeding mares as early as two years (in some cases earlier), but it is healthier for the young stallion if breeding does not begin until four years. In both cases, early breeding may affect growth. A mare may continue carrying foals until they are in their late twenties. A stallion may continue breeding mares into his twenties, although his potency will decline.
Picture of embrionic sac
Average Gestation Period
The gestation period, or time from conception to birth is approximately 340 days or 11 months. Some mares will be inclined to foal earlier or later than the average, and breeders will get to know these tendencies.Mares are generally more receptive to stallions during the late spring and summer months, although they may have oestrus cycles approximately every three weeks, year round. However, breeders who wish to manipulate the breeding cycle so foals are born earlier in the year (most commonly race horses) will use lights to simulate the longer days of spring and summer. The artificial daylight stimulates the mare's reproductive hormones, and they can be bred earlier and in turn will foal earlier in the year.
Checking For Pregnancy
Beyond the absence of an oestrus cycle, mares may not show any visible signs of pregnancy for the first three months. Pregnancy can be confirmed by ultra-sound after approximately two weeks after the breeding took place. Blood and urine testing can be done two to three months after conception. Alternatively, a veterinarian may be able to manually feel changes in the mare's uterus approximately six weeks into the pregnancy.
It's important to have the mare checked by a veterinarian early in the pregnancy for her health and the health of her foal. Horse twins are rare, but can lead to the mare aborting.. It's not unusual for a mare to lose a pregnancy, so it's recommended to ultra-sound, blood or urine test again after about three months. 
Later Stages of Gestation
After about three months the foal will be developing rapidly and start to look like a small horse. After about six months, the mare may start to be visibly pregnant. Mares that have foaled before may show an expanding belly sooner than a maiden mare. Over the remaining months, the mare's belly will continue to grow as the foal approaches its foaling date..
Position of foal
After about 315 days of pregnancy, the mare will start to show signs of foaling. The yellowish fluid from her udder will turn into the first milk or colostrum. The udder may drip, and the muscles around her tail head may become more relaxed. Her belly may appear to drop, as the foal positions for birth. At this point, birth is imminent and the mare must be checked frequently for signs of foaling. Shortly before birth the mare will appear restless, may paw the ground and check her sides (similar to colic symptoms). She should be stalled in a large clean stall, preferably bedded with straw. The mare may lie down and get up repeatedly and may give birth standing or lying down. First, the amniotic sac may be visible, and then the foal's front hooves and nose. The foal is normally birthed within a few minutes at this stage.
Foals are able to stand within minutes
Foals are able to walk and run a few hours after birth. They may nibble grass, concentrate or hay within a few days after birth, although their mother’s milk will be the main source of nutrition.  They may be weaned from their mothers as early as three months after birth, although many breeders choose to leave mares and foals together longer