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A Window on High-Risk Pregnancies
Article and Photos by Kim and Kari Baker

     Diagnostic ultrasound has assisted the medical doctor for many years in evaluating the fetus in pregnant women, aiding in specific diagnoses and treatments of gestational complications. Now expectant mares can reap some of the same benefits. Problem pregnancies are not uncommon in the mare and ultrasound can provide a window on the developing foal, enabling the practitioner to determine gestational age, detect multiple gestations and assess fetal well-being.

     The effect of regular ultrasound exams on perinatal outcome in low risk mares has not yet been justified, therefore the role of routine screening remains debatable. However, mares that have a history of stillborn foals, twin pregnancies or have been ill are good candidates. Any ultrasound image should be evaluated with respect to the mare's history and a physical exam. Signs of trouble in a high-risk pregnancy may include vaginal discharge, excessive abdominal size, and premature udder development, lactation or waxing. The presence of pain and apathy may also indicate gestational complications.

     The examination is performed primarily from the underside of the mare, scanning the belly from in front of the udder to just behind the sternum and up along the sides almost parallel with the stifle. Clipping the abdominal hair and lubricating the area with coupling gel will aid the veterinarian in capturing a clearer image. The veterinarian will want to examine the placenta and umbilical cord as well as the fetus, but due to the large size of the fetus during late gestation only portions of the foal can be observed at one time. The six valuable indicators or measurements drawn upon to determine fetal health are heart rate of the fetus, fetal movement, aortic diameter, placental thickness, placental attachment to uterus and amount of amniotic and allantoic fluid.

     A good indication of whether a foal will be depressed at birth or is experiencing stress is the fetal heart rate. A depressed neonates heart rate will dip below 70 beats per minute, while a stressed neonate's heart rate may race above 90. A change in fetal heart rate can furthermore indicate a pending abortion, particularly if there is also an increase in fetal fluid within the placental membranes.

     The foal's position and movement can be appraised and checked periodically during the last stage of pregnancy. A healthy foal moves rather frequently. While it makes the scan more difficult, an active foal is a good sign. Also, the muscle tone of the fetus can be determined by the ability of the fetus to move the extremities. The fetus should assume the correct anterior position by about the eighth or ninth month of gestation. Even though it can still roll from side to side, it usually won't make a 180-degree turn around. Knowing in advance that the a foal is mal-positioned such as in a breech presentation, will aid you and your veterinarian in making preparations for the difficult birth. It is also important to understand that other than determining the overall position of the foal, it is not possible to always predict whether there will be dystocia caused by a head or limb turned back or a problem with the mare such as uterine inertia.

     Fetal Weight can be estimated through aortic measurements, giving the practitioner a sense of the fetus's gestational age. The length of gestation and the actual gestational age of the fetus may not necessarily be one and the same. Illness or poor nutrition may slow development of the fetus, and there has been some speculation that an early embryonic rest for up to 30 days may occur under special conditions resulting in a foal that appears younger than expected.

     If a thickened placenta is discovered during the exam, placentitis or fescue toxicosis may be the cause. To examine the placenta the veterinarian can place the sector scanner against the mare's lower abdomen or to examine the placenta in the region of the cervix he can place a probe in the mare's rectum. An examination of the cervical region is notably important if the mare has a vaginal discharge or is suspected of having placentitis, an infection present in the uterus. Based on his findings, the veterinarian may prescribe antibiotics, uterine relaxants, and hormone supplements or advise the mare be removed from pasture or a change of hay be made if it is possible that the mare is feeding on an endophyte infected forage. Immediate treatment of the foal at birth with antibiotics may also be recommended in cases of placentitis.

     The measure of placental attachment can be an indication of good fetal health or forebode an impending abortion. Although there is no treatment for any degree of placenta separation, placentitis is often the cause and the veterinarian may again recommend the use of hormone supplements and antibiotics or in urgent cases he may discuss a planned induction. In these critical cases prompt emergency care can then be provided for the mare and foal.

     The fetus is contained within two sacs or membranes. The fluid enclosed in the outer fetal sac is known as allantoic fluid while the inner sac contains amniotic fluid. Fetus well-being can be evaluated by measuring the clarity and amount of fluid in pockets surrounding the fetus.

     The fluid should be relatively clear. If it has a lot of floating particles there may be an infection of the placenta or bleeding into the fetal fluids. This is a good sign to further evaluate the pregnancy with blood tests, cultures and a vaginal exam.

     The abnormal accumulation of fluid within either of these fetal membranes is known as hydrops. When excessive uterine fluid is discovered in one of the sacs, a fetal defect may also be uncovered. There are a multitude of causes including fetal cardiac disease, congenital abnormalities, congenital infections, fetal anemia, neurological disorders or umbilical dysfunction due to stricture or torsion.

     Hydrops usually occurs after seven months of gestation and a gross abdominal distention – much more than her stage of pregnancy would suggest – will be noted. The distention occurs over a ten to fourteen day period and the mare may go off feed and show signs of discomfort and difficulty in breathing while lying down. Unfortunately inducing labor is frequently the only remedy and loss of the foal is almost certain if the pregnancy is short of 10 months. Due to the sudden loss of a substantial amount of fluid, shock is probable and the mare should be hospitalized so that she can be maintained on intravenous fluids. Hydrops also predisposes the mare to prepubic tendon rupture, uterine rupture, and abdominal muscle rupture. Too little fluid can also indicate trouble. It is usually evidence of a compromised pregnancy due to a malfunctioning placenta and asphyxia. Such neonates are often starved of oxygen and smaller than normal.

     Any pregnancy that reveals only one of these abnormal parameters is two and half times more likely to have a poor outcome than one with a perfect exam. Thus, a timely ultrasound exam, identifying the high-risk pregnancy before delivery occurs, allows the owner and their veterinarian a window of opportunity to take the needed precautions.

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