The equine foetal membranes, or placenta, are the structures that enable the mare to support a healthy pregnancy as it develops. Within the uterus the foetus is surrounded by the innermost layer of membrane, called the amnion. This is a whitish, shiny layer enclosing the amniotic fluid.

The combined middle and outer layers of the placenta are called the allantochorion, which surround and contain the allantoic fluid. The central allantoic membrane layer is pale and shiny, while the reddish and velvety-textured outer chorion directly contacts the lining of the mare’s uterus.

This arrangement of layers of fluid and membranes enclose, nourish and protect the developing foal. There are numerous tiny chorionic blood vessels which contact the uterine lining closely enough to allow oxygen and nutrients to be passed from the mare’s bloodstream into the placenta.

The umbilical cord connects the chorion to the foal. This arrangement enables the umbilical blood vessels to carry nutrient and oxygen-rich blood to the foal, and remove wastes.

Tissue damage

A typical equine umbilical cord is 40cm-80cm long and contains three blood vessels (a pair of umbilical arteries and a single umbilical vein). Another tube called the urachus runs through it. This structure drains the foal’s bladder into the allantoic fluid until it is born.

The umbilical cord usually has several gentle spirals along its length. As long as these twists are not numerous or dramatic enough to compromise the vessels within the cord, they are considered to be normal.

Excessive twisting of the cord can reduce or prevent blood flow along it, resulting in foetal abnormalities and death.

Sometimes the cord can get tightly wrapped around the foal’s leg or neck. The resulting tissue damage and swelling can lead to foetal death due to a lack of blood flow and oxygen. Cords that are greater than 80cm in length are more prone to twisting.

It’s important to be aware that lots of other conditions can also cause a developing foetus to die, including foetal malformations, twinning, inflammation (placentitis) caused by bacterial infection, or viruses such as equine herpesvirus 1 and 4. If the foal dies within the uterus before it reaches its full size, it will stop moving and instead bob around in the amniotic fluid before it is expelled from the uterus (aborted).

This lack of normal movement can lead to the umbilical cord becoming tightly wound around on itself. It may therefore appear that the twisted cord was the cause of the abortion, when in fact it occurred after the foal died.

If the foal was alive when the excessive twisting occurred there will be some evidence of swelling and bruising within the cord, whereas this won’t be the case if the twisting only developed after death.

It’s extremely disheartening after all the work and resources that go into getting a mare successfully in foal only to find she has passed an aborted foetus several months later. The temptation is there to just try and put it behind you, but a post mortem examination can help to confirm the most likely cause of the pregnancy loss.

Laboratory assessment

Not every abortion could have been prevented, but it’s almost always worth making the attempt to send the aborted foetus and afterbirth to the laboratory for assessment.

Hopefully the information gained can not only provide some answers for what might have gone wrong, but also help in some cases to formulate a plan to reduce the risk of pregnancy loss in subsequent breeding seasons.