The placenta is a remarkable organ — a complex, highly convoluted, six-layered structure made up of foetal and maternal tissues.
For the first 40 or so days of pregnancy there is no stable attachment between the embryo and the uterus. A substance called histiotroph (sometimes termed uterine milk), produced by the uterine glands, bathes the embryo and sustains its development.
The placenta starts to develop at around 40 days and fills the whole uterus by 85-90 days. By 150 days it is fully established.
The area of contact between foetal and maternal tissues continues to increase up to birth. It allows exchange of nutrients, oxygen and hormones from mare to foetus, and passage of waste products and hormones from the foetus to mare.
The chorionic side of the placenta (fetal part) is closest to, and is a mirror image of, the lining of the mare’s uterus. Tiny microvilli (protrusions) link together with the mare’s uterine lining, giving this side a red, velvety appearance.
Endometrial cysts and scars, plus areas of fibrosis or where there has been poor contact between maternal and foetal tissues, lack this velvety appearance. Once the placenta has been passed, at birth, the chorionic side discolours rapidly.
The allantoic side of the placenta, in contact with the foetus, is a smooth, pinkish-grey colour. White blood vessels traverse its surface.
The amnion is the translucent, white sac that surrounds the foetus and attaches to the umbilical cord. This is the layer of membrane that you usually have to clear from the foal’s muzzle as he is delivered.
Studies suggest that the first pregnancy in effect “primes” the uterus for maximal placental development on subsequent pregnancies, which explains why first foals can sometimes be smaller.
The area of normal, healthy contact between maternal and foetal placental tissues is the single biggest factor affecting foetal growth. The foal’s size can thus be compromised in the case of placental insufficiency — where the placenta has not developed and functioned normally.
Several things can affect placental size, including:
- The mare’s size. In general, a bigger mare has a bigger uterus.
- The mare’s age. Degenerative changes of the uterus associated with ageing can reduce the number of healthy uterine glands, in turn reducing uterine milk secretion. Cysts and fibrosis will also reduce the area for development of normal healthy placenta.
- Inadequate nutrition, particularly in the period of rapid placental development between 40-150 days. There may, however, be some increase in efficiency of the placenta to compensate for reduction in size. This may also compromise the placenta and in turn the foetus.
- Maternal illness.
Any problem that reduces the size or efficiency of the placenta results in intrauterine growth retardation (IUGR) of the foal. These pregnancies can be prolonged. Foals affected by IUGR are small and have reduced muscle and fat, often suffering other developmental problems.
Another risk is placentitis, an infection that can develop in the placenta within the last three to four months of pregnancy. This is usually a result of bacteria entering the vagina and passing into the uterus, where they multiply rapidly.
The first signs of placentitis that most people notice are premature development of the mammary glands and a vaginal discharge or a sticky “mat” on the underside of the mare’s tail. In cases where the infection is localised in an area of the placenta called the cervical star, the condition is confirmed by a transrectal ultrasound examination. The degree of thickening of the placenta, the appearance of foetal fluids and placental separation can then be assessed.
Transabdominal ultrasound examination can be used to assess the size of the foetus and its heart rate, patterns of activity and sleep — all good indicators of wellbeing.
Sadly, placentitis is a difficult condition to treat and can frequently result in abortion. Treatment is based on the use of antibiotics and altrenogest (known under the brand name Regumate) and, in some cases, anti-inflammatories.
Is it complete?
When the foal is delivered the umbilical cord breaks, causing collapse of the blood vessels on the foetal side of the placenta and separation of the placenta from the lining of the uterus.
Uterine contractions, stimulated by oxytocin release, spread from the tip of the uterine horns towards the cervix. This turns the placenta inside out and squeezes it out of the uterus.
As it passes out through the vulval lips, its weight hastens the process of expulsion. It is useful to tie up the placenta at this stage to stop the mare standing on it and tearing it.
Never pull the placenta, as this can lead to tearing of the tissues, retention of the microvilli or haemorrhage — which can result in potentially life-threatening complications for the mare.
After the placenta has been passed it should be carefully laid out and checked for completeness, ideally in an area that can be hosed down and disinfected afterwards. There should be two “horns” (see image, below left) — a larger, pregnant horn, often with a thicker tip, and a non-pregnant horn, the tip of which is most commonly retained.
Both sides should be examined, although it is difficult to interpret changes in colour of the chorionic side as it can rapidly become discoloured with storage.
Keep the placenta and consult your vet immediately if you are concerned that a piece is missing or there is something unusual about it.
The placenta should be passed within three hours of foaling, usually within an hour. The incidence of placental retention exceeding three hours varies between breeds, occurring in around 5% of thoroughbreds, a quarter of heavy and draft mares and half of Friesians.
The high incidence of the condition in Friesians is thought to be influenced by in-breeding. If you are expecting a foal from a breed with a high incidence of placental retention, discuss the best way to manage the mare with your vet before she foals.
Retained placenta is treated with injections or infusions of oxytocin, along with systemic and intrauterine antibiotics and uterine lavage.
It is vital that the condition is treated without delay. Septic metritis (infection of the uterine wall) and accompanying laminitis can develop rapidly as a complication, threatening the mare’s life.
Ref: Horse & Hound; 14 April 2016