Unless you examine many placentas (as vets do), the anatomy may be confusing. However, it is helpful for horse owners to conduct a preliminary examination of their mare’s placenta and the umbilical cord after foaling.
Most mares pass the placenta within 30 minutes of foaling and it should be passed in whole within 2 hours of foaling. A placenta is considered retained if it has not passed within 3 hours of foaling. It is critical that there not be any piece of placenta left in a mare. This can cause life-threatening infection and must be discovered and removed by a vet right away.
The umbilical cord of light horses is usually between 30 and 80 cm in length but can be a meter long. Visual evaluation of the umbilical cord may be helpful in determining fetal death or other health problems in the foal.
The placenta is made up of two sheets or membranes. The innermost surface that surrounds the fetus is called the amnion. It is a pale-colored thin-veined membrane. The outermost surface that joins the placenta to the uterus is the velvety red chorio-allantoic membrane.
When you look at a placenta of a foal that has recently been born, the placenta has been turned inside-out in the birth process. The red chorionic surface will be inside while the slick, pale, glistening allantoic surface is outside. This inversion occurs when the foal, still contained within the amnionic membrane, breaks through the chorio-allantois at the cervical star (a pale, weak area of the placenta that lays against the cervix during pregnancy that has no villar attachments).
The foal then moves into the cervix and vagina, pulling the chorio-allantois by its attachment to the umbilical cord, and everting the chorio-allantois through the hole at the cervical star.
Start by laying the placenta out in an F shape. The 2 arms of the "F" correspond to the two uterine horns and the long uterine body ends in the cervical star. While the pale, glistening allantois surface is still outermost, check for any tears or abnormalities.
The most common area for tears is the tip of the wrinkled non-pregnant horn, so check that area carefully. The non-pregnant horn is wrinkled because it was not stretched as much during pregnancy as the pregnant horn.
If you are sure that the placenta is complete, turn it inside out (like a sock) starting at the cervical star, exposing the velvety red chorionic surface, which was attached to the uterus.
Lay the placenta out in an "F" shape again. Notice the normal huge tear at the cervical star through which the foal exited at birth. Follow the arms of the placenta to the tips of the horns and see that they are complete.
A normal placenta may show slight thickening of the tips of the uterine horns, and small bare spots randomly scattered over the red surface. The presence of yellow-brown staining of the placenta and foal is called meconium staining and is a reliable indicator of fetal stress and an at-risk foal.
Twists (torsion) in umbilical cords is common, especially in longer umbilical cords. It is common to see five or six twists of the umbilical cord with a normal healthy foal, while 10-20 twists might be considered to be the cause of fetal death. Local areas of bleeding and swelling of the umbilical cord are observed in some cases. Whether twisting is a cause of fetal death or a result of fetal struggle remains unclear.
The hippomane is an olive colored, free-floating, meaty slab of tissue that during pregnancy is within the allantoic cavity. The source of the hippomane is not entirely understood. It may start as an out-pocketing of the allantois, which eventually separates to float free in the allantoic cavity. I was taught that it is a collection of fetal cells and urine.
Tips for safety & Success
If you are not confident in performing this skill or otherwise have any doubt about whether your mare passed the entire placenta, call your vet immediately to discuss your findings and concerns.
Do not attempt to manually remove the placenta unless you cannot get veterinary help. Do this under veterinary guidance only.
Monitor your mare closely paying particular attention to her attitude, appetite and temperature. Save the placenta and umbilical cord for your vet to evaluate.
If you have foaled out a broodmare, or a mare that you plan on breeding regularly, it is good practice to obtain an accurate weight of the placenta. Put the placenta in a plastic bag and weigh it on a postal scale. The normal placenta of a near-term light horse mare should weigh about 11% of the body weight of the foal or approximately 5.7 kg (12.5 lbs) for a 1000 lb horse. The weight of the placenta can change when there is thickening or edema, which is useful information when addressing reproductive issues.
Helpful Outside ResourcesCredible Equine Health Information on the Internet