What is the placenta called after it leaves the mare's body?

The placenta is referred to as the afterbirth once it has been expelled from the mare's body. This term specifically indicates the tissue that provided nourishment to the developing fetus during pregnancy but is no longer needed after the foal is born. The afterbirth signifies the completion of the birthing process and is a natural part of equine reproduction. Understanding this term is crucial for anyone involved in horse breeding and care, as it highlights a significant stage in the reproductive cycle.

The other terms, while related to reproductive structures, do not accurately describe the placenta post-expulsion. "Placenta" refers to the organ itself during pregnancy, while "maternal sac" and "fetal membrane" are more general terms related to the protective layers surrounding the fetus. Therefore, "afterbirth" is the appropriate term to use in this context.