Horse Embryo Development

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The time of conception until the birth of a baby foal is known as equine gestation, and in healthy mares averages 340 days. Foals born too early, around 315 days, are considered premature, and need consistent care to survive. Foals born before 300 days usually do not survive, as they are too underdeveloped to properly breathe, move and eat.

From a Speck to Legs

Once a mare is pregnant she will no longer come into heat, an almost-monthly cycle when the mare is receptive to procreation efforts from stallions. While visual changes won't begin to occur in the mare for the first 100 days, she will begin to eat more and may display signs of moodiness or defensiveness. In these early days it is not uncommon for the mare to "slip", or miscarry; sometimes this happens without the owners noticing.

A brand new embryo appears to be a small, perfectly round sphere just over one-tenth of an inch in diameter. In the middle of the sphere is a tiny pale speck: This infinitesimal grouping of cells will grow and eventually form the fetus. The remainder of the outer sphere becomes the placenta, a protective sac that keeps the baby horse nourished.

Just after fertilization, the embryo moves about the womb of the mare, looking for a safe place to grow. About two weeks into the pregnancy it will come to rest on the side of the mare's uterine lining, and fixate there. It is at this time that cell division begins.

During cell division the embryo is nourished by a yolk sac; this yolk sac eventually will be absorbed into the umbilical cord of the fetus. At this time he will receive food and oxygen from his mother. Around three weeks into gestation, the foal's heartbeat can be detected for the first time. When his umbilical cord is fully formed, around 40 days into gestation, the embryo is now known as a fetus.

Growing in Leaps and Bounds

Not long after the embryo develops into a fetus, the sex of the foal can be determined by ultrasound. Hooves also form at this time, covered in feathery-looking soft eponychium, that protect both the hoof and the mother's uterus during pregnancy. After birth, the eponychium fall off painlessly.

During the initial half year of gestation the foal doesn't grow much; after six months he is roughly the size of a cat. Inside the womb, he looks very horse-shaped, and his mane and tail have begun to form. At this time his growth increases rapidly: In the remaining five months the fetus will go from 25 to around 125 pounds.

By the 270th day the fetus resembles a tiny horse. He is covered in hair, and sports a tiny tail. He's about the size of a German shepherd dog, and is gaining a pound a day. This time is crucial in the mare's pregnancy, as she gets closer to the due date. The little horse's lungs are still forming as he prepares to exit the womb.

Three hundred days into gestation the mare's abdomen will drop, her udder will begin discharging milk and her vulva will lengthen and lose tension. From this point until delivery she will need to be watched every day. Prior to giving birth the mare will sometimes attempt to head off alone, and she may lay down often. This is a sign of beginning contractions, and indicate that soon the newborn foal will emerge.

What Can Go Wrong?

Problems during a mare's gestation can manifest at any time, so she should be carefully observed throughout her pregnancy. Ten percent of mares will slip, or simply reabsorb their fetuses, and 6 percent will spontaneously abort or give birth prematurely. Three percent of mares experience stillbirth, when the foal appears to be perfectly healthy, but is born dead. Sometimes stillbirth occurs during dystocia, or foaling difficulties. These cases are serious, but rare, and in the majority of cases -- roughly 82 percent -- embryos are successfully foaled as wet, fuzzy and hungry newborn horses.

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