Prenatal Development of a Horse

On average, equine gestation lasts 11 months or 320 to 365 days. Although fertilization rates can be as high as 90 percent, approximately 30 to 40 percent of pregnancies are terminated within two weeks. During breeding, the mare's egg travels toward the sperm with the aid of the oviduct's cilia, which move rhythmically to bring the sperm and egg together. Fertilization occurs in the fallopian tubes, passes through the oviduct and finishes its journey in the uterus within six to seven days.

  1. Post Fertilization

    • The mare's uterus provides the embryo with protection and nourishment during its development and until birth. The equine uterus possesses unique characteristics unlike that of other domestic animals, which are essential to the successful reproduction of the species. It has the ability to rid itself of endometrial inflammations, as well as the capability to defend against considerable bacterial contaminations. After fertilization, the embryo begins to develop a capsule, known as the acellular glycoprotein. Although its function isn't completely understood, it's believed to perform a vital role in establishing and maintaining a healthy pregnancy, as well as survival of the fetus. Between the 12th and 15th day, the embryo begins to move independently within the uterus and in response to the mare's natural contractions. This process leads to essential toning of the uterus and "fixation" of the embryo.

    Early Development

    • Approximately 16 days following fertilization, the embryo stops moving and "fixes" or attaches itself to one of two horns within the uterus. A yolk sac provides essential nutrition to the developing embryo in the early stages, which eventually becomes part of the umbilical cord. The fetus's heartbeat can typically be heard by the 21st day. Within approximately 40 days, the yolk sac has disappeared, the umbilical cord is fully formed, membranes that line the placenta are complete and the amniotic fluid surrounds the growing fetus.

    Fetal Growth

    • On approximately the 45th day, the placenta exchanges essential developmental activities for the rapidly developing fetus, such as waste elimination, respiration and the dam's nutrition intake. After nearly two months, the fetus is approximately 2 1/2 inches long. Although its hooves are beginning to develop, the head remains tucked in and is hairless. During the six months that follow fertilization, fetal growth is relatively slow. Within 120 and 150 days, the fetus gains nearly 1 lb. every 10 days, is the size of a rabbit and has eyelashes and hair on its eyelids, muzzle and chin. By approximately the 270th day, the fetus is the size of a German shepherd. It resembles a foal and has hair over its entire body, including a small amount on its tail. At this stage of growth, the foal lays on its back in preparation for birth. In the last week of growth, the foal's lungs are fully developed, its legs are straight, its teeth have grown through the gums and its head rests between the forelegs for delivery. The majority of fetal growth occurs in the last three months of pregnancy when it will develop from 37 to 44 lbs. at nine months to approximately 125 to 132 lbs. at birth. Although genetics play a role in the foal's size, the mare's uterine capacity is a large determining factor.

    Risks and Pregnancy Termination

    • Primary causes of equine fetal abnormalities include infections within the cervix and uterus, which results in death. The most common bacterial infections found in equine aborted fetuses are equine herpesvirus, rhinopneumonitis, Aspergillus spp, Streptococcus spp, Escherichia coli, Pseudomonas spp, Klebsiella spp and Staphylococcus spp. Spontaneous abortions for infectious reasons generally occur within seven months of gestation and represent between 10 and 15 percent of all terminations. Other causes include factors related to the mare's overall physical health, environment, ingestion of poisonous plants during critical stages of pregnancy, improper nutrition, deficiencies in vitamins and minerals, and inherited genetic and chromosomal weaknesses.