The placenta is the temporary organ in the mare that forms during pregnancy to support the foal while it is in the mare. The dictionary defines placenta previa: the condition in which the placenta is implanted in the lower segment of the uterus, extending to the margin of the internal os of the cervix or partially or completely obstructing the os. Syn: placental presentation.

In equine medicine this term is used for the condition where the placenta prematurely detaches from the uterus and is presented at the vulva without rupturing to release the foal. It is referred to as red bag because the bright red outer surface of the placenta is presented first during birth and is the first indication something is wrong. This article discusses this condition, presents scientific reports, and provides links to other discussions.

The association of the ultrasound finding of "the placenta covering the cervix" and "premature detaching and presentation of the placenta with no rupture at the time of birth" is not well established in horses. This is important because much of the research in humans involves the condition of the placenta covering the cervix. As you read the research in humans be aware that the applicability to horses is unknown.