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High Risk Pregnancy in Mares

By: Dr. Mary Rose Paradis

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Conditions associated with high risk pregnancies or foals can be due to maternal problems, neonatal problems or conditions of labor or delivery. They are as follows:


  • Premature udder development and colostral leakage
  • Purulent vaginal discharge
  • Fever
  • Colic surgery
  • Endotoxemia
  • History of foal with isoerythrolysis
  • History of foal with maladjustment syndrome
  • Poor nutritional status
  • Prolonged transport before parturition


  • Meconium-stained fluid
  • Premature placental separation
  • Placentitis
  • Twins
  • Orphan
  • Inadequate colostrum ingested
  • Immaturity/prematurity
  • Trauma
  • Delayed standing or nursing

    Labor and Delivery

  • Premature parturition
  • Prolonged gestation
  • Prolonged labor
  • Induced labor
  • Dystocia
  • Early umbilical rupture
  • Cesarean section

    Early mammary development and dripping of colostrum (before 320 days gestation) are signs of impending premature birth. The differential diagnosis for these signs should include possible twinning and placentitis. Twinning can sometimes be ruled out on the basis of a transabdominal ultrasound, which allows for non-invasive monitoring of the late term fetus. Images of the late term foal can usually be obtained at the most ventral area of the mare's abdomen. The most recognizable feature of the foal is his thorax where the shadows of the ribs can be detected. From this point the beating heart can be observed. Information that can be gained from the ultrasound include the presence of twins, the approximate size of the fetus, the fetal heart rate, the placental thickness and a gross evaluation of the allantoic fluid volume.

    Fetal heart rates vary between 70 to 110 beats per minute. One evaluation of fetal heart rate only gives you a moment in time picture. Repeated monitoring is a better predictor of fetal health. Persistent fetal tachycardia (over 110 beats per minute) is an indicator of fetal distress. The presence of two different fetal heartbeats confirms the presence of twins. It is sometimes difficult to know for sure that you are looking at two separate heartbeats because an active foal moves around the abdomen constantly.

    The vulvar discharge and fever in the mare are compatible with placentitis. Both bacteria and fungal organisms can infect the placenta. It is felt that the route of entry for infectious organisms is generally ascending through the cervix, though it is possible that infection could have occurred at breeding or through a hematogenous (blood) route. Beta-hemolytic Streptococci and Escherichia coli are the most commonly cultured bacteria from a placentitis, while Aspergillus is the most incriminated fungal organism. The foal is at risk of developing an in utero infection in mares with placentitis. The bacteria can pass through the placenta to infect the foal. This infection may weaken or kill the foal. The inflammation associated with the infection may result in a premature delivery of the foal.

    Prematurity in the foal can be accompanied with myriad of problems, in particular, problems associated with the respiratory tract. Surfactant, a phospholipid compound, is produced in the lungs of foals in late gestation. Surfactant acts to decrease surface tension in the alveoli and aids the inflation of the lungs. A total lack of surfactant leads to increased respiratory effort, pulmonary edema and hyaline membrane formation. This is generally known as respiratory distress syndrome, and equine neonatalogists have had little success in saving the truly premature foal.

    Prematurity has been defined as a gestational age of less than 320 days. This technical division between prematurity and maturity does not stand up in all cases. Readiness for birth is a better concept in looking at prematurity and immaturity. A foal induced at 330 days gestation may not be ready for birth even though it is more than the technical 320 days. If it were not going to be born naturally until 350 days gestation, then it would be at high risk for respiratory distress syndrome. Whereas, this foal born at 312 days gestation was ready to be born in regards to its pulmonary function. It is hypothesized that chronic maternal stress, such as twinning or placentitis, actually acts to hasten lung maturity.

    Small size and the laxity of flexor tendons are usually signs of immaturity or dysmaturity in a foal but may also be seen in the post-mature foal (gestation over 360 days). Post-mature foals are often small and emaciated. A thin body condition is probably a result of placental insufficiency or intrauterine growth retardation (IGR). The foal is essentially starved in the uterus because the maternal nutrients are not able to cross the placenta. Gestations over 360 days should be considered a high-risk pregnancy.

    Lack of colostrum from the mare or failure to nurse within three hours of age are potentially high risk factors. If frozen colostrum from another mare can be obtained, then it is the best way to give this foal passive immunity. Otherwise, plasma transfusions are the best alternative. Prophylactic antibiotics are important in case there is a pre-existing infection.

    There is no way to really predict malpositioning of the foal during delivery. While in the uterus the foal is quite active and changes position frequently. During the first stage of labor, the normal foal assumes the position of front feet first, head on carpus. Any deviation from this position may cause a delay in the progression of the delivery. During the delay the foal may become distressed. This distress will sometime lead to expulsion of meconium (the foal's first fecal matter) while the foal is still in the uterus. In the foal's attempts to breathe it may breathe in the meconium contaminated amniotic fluid. This is a serious complication that could lead to severe respiratory problems in the newborn.

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