Placenta Evaluation

Looking at the connection between mare and foal can give you advance warning of health risks.

The placenta is the connection between the developing fetus and the mare and provides the means for obtaining nutrition and gas exchange. It can offer valuable insight into the health of the newborn foal. As soon as the mare passes the placenta, you should evaluate it.

The placenta is normally passed within three hours after foaling. Once passed, the placenta should be immediately removed from the foaling area and rinsed. Disposable plastic or latex gloves should be worn during handling and examination. A bathroom scale works to determine the weight of the placenta. Under normal circumstances, the weight of the placenta will be approximately 11 percent of the body weight of the foal. An increased weight may be the result of edema.

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The placenta consists of three primary components: the outer placental membrane or chorioallantois; the inner placental membrane or amnion; and the umbilical cord. In a normal foaling, the thicker outer placental membrane is almost always turned inside-out. For examination, the outer placental membrane should be turned right-side out and then spread out on a clean flat surface. The allantoic (side toward the foal) surface is smooth and pink and blood vessels are visible coursing along its surface. The chorionic (side toward the uterus) surface has a brick- red velvety appearance due to microcotyledons. Microcotyledons are the microscopic villi or finger-like projections that attach the placenta to the uterine lining. The membranes might be laid out with the chorioallantois in an “F” or “Y” shape, with the two horns of the placenta forming the arms of the “F” or “Y” and the body of the placenta forming the base.

One of the first things that may be noted is that the tips of the placental horns differ significantly in size and thickness. The larger horn housed the fetus and is referred to as the pregnant horn. The tip of the pregnant horn is always thicker than the tip of the smaller horn. Examine the placenta to determine whether it was passed intact or if a piece is missing and potentially still inside the mare. By far the most likely portion of the placenta that may be retained is the thin tip of the nonpregnant horn. Even a small piece of placenta in the uterus poses a health threat. The area of the body of the placenta that was in direct contact with the cervix of the mare will be pale in color. This area is referred to as the “cervical star.” Ascending bacterial infections that pass through the cervix can result in thickening or the presence of a mucus-like substance in the area around the cervical star. If a placental infection is present, the foal might have been exposed to pathogenic microorganisms prior to being born. The thin, white amnion should be examined next. The primary abnormality that might be noted in the amnion is a yellow-orange discoloration due to meconium staining. Stress to the fetus prior to birth can result in premature passage of fecal material (meconium). Meconium staining can be an early warning sign that the foal is compromised.

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The umbilical cord will almost always have a mild to moderate degree of twisting present. However, excessive twisting of the cord can result in decreased blood flow from the placenta to the fetus and other problems. Evaluation of the placenta is a valuable procedure that can positively impact the health and welfare of both the newborn foal and the mare. It is recommended that your veterinarian be contacted for an initial lesson in field evaluation of the placenta and that he or she be notified if abnormalities are identified in subsequent placental examinations.