Horse Genetics: Characteristics of the Equine Placenta

The placenta provides protection for the fetus and regulates the fetal environment. Previous work in the United Kingdom has examined placental parameters associated with normal births. A study was undertaken in the Central Kentucky area by veterinarians from the Hagyard Equine Medical Institute to examine placental parameters associated with foals making a normal transition to extra-uterine life. This study comprised observations from 168 foalings during a two-year period on three farms. Information was collected on factors related to the mare, the foal, and the placenta, as illustrated in Table 1.

Table 1. Placental parameters associated with foals making normal transition to extra-uterine life.

Category	Variable	Number	Mean	Standard Deviation	Range
Mare	Age of Dam (yrs) Number of Registered Foals Time to Pass Placenta (min)	168 155 75	12.7 6.0 50.3	5.6 4.1 52.7	5-26 0-15 5.0-357.0
Foal	Gestational Age (days) Foal Weight (kg) Time to Stand (min) Time to Nurse (min)	167 157 150 136	342.0 54.1 66.4 112.7	8.3 6.3 34.1 48.3	312-362 29.5-74.5 15.0-260.0 35.0-290.0
Placenta	Total Umbilical Length (cm) Amniotic Cord Length (cm) Chorionic Cord Length (cm) Number of Cord Twists Number of Amniotic Twists Number of Chorionic Twists Placental Weight (kg) Chorionic Weight (kg)	165 85 85 162 82 82 59 141	65.0 32.6 30.9 0.94 0.54 0.49 7.1 4.1	19.0 9.0 11.5 1.4 0.84 0.82 1.4 1.0	30.5-183.0 15.5-65.0 9.0-62.0 0-7 0-5 0-4 3.6-11.4 2.3-7.3

Source: Observations from 168 foalings during a two-year period on three Central Kentucky farms.

To gain insight into the normal parameters, researchers in the past have used a set of exclusion criteria to eliminate potentially abnormal placentas. These were mostly associated with abnormal foaling events (i.e., abnormal delivery, more than two hours for the foal to stand, or more than four hours for the foal to nurse). The study found that there were not significant differences in the placental parameters between those classified as normal and abnormal based on foaling events. It suggests this arbitrary method of classification may not in fact separate normal and abnormal placentas. Different methods of classification based on placental characteristics, such as the presence of placentitis, should be examined in the future to separate normal and abnormal placentas.

The average times to stand and nurse, 66.4 and 112.7 minutes respectively, were similar to results recorded in previous studies. There was not a significant correlation between foal weight and the time to stand, agreeing with previous results. However, foal weight was significantly correlated with gestational age, allantochorionic weight, and the age of the dam.

Long umbilical cords have been associated with three outcomes: strangulation, excessive cord twisting, and necrosis of the cervical pole of the allantochorion. These outcomes lead to increased risks of abortions and stillbirths. One of the aims of the current study was to examine factors that influence umbilical cord length. The influences of 49 different factors on total umbilical cord length were examined; two were found to be significant after controlling for other variables. As mare age increased, the length of the umbilical cord also increased. Also, as the length of the nonpregnant horn increased, umbilical cord length also increased. This could be due to an increased ability of the foal to move within the uterine environment because of the parity of the dam (i.e., greater fetal kinesis).

Mares were divided into three age categories: less than 8 years of age, 8 to 14 years of age, and more than 14 years of age. There were significant differences in some of the parameters by mare age. Mares more than 14 years of age had longer gestational lengths, heavier allantochorionic portions of the placenta, and shorter time to a foal’s first nursing. Foal weights were lowest among those mares less than 8 years of age.

This study provides information on the parameters associated with a normal foaling in the Central Kentucky area. In future, studies on correlations between in utero measurements and measurements at birth will provide veterinarians with valuable information for diagnosing problems earlier in fetal development.

By Karin Bosh, Graduate Research Assistant - Maxwell H. Gluck Equine Research Center, University of Kentucky; and Dr. Luke Fallon - Hagyard Equine Medical Institute, Lexington, Kentucky.
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