Monitoring Fetal Well-Being
Fetal monitoring in horses is similar to the methods used for humans -- ultrasound, EKG machines, and blood tests on unborns all are part of monitoring the health of the fetus. Advances in these areas for equines have allowed for detection of problem pregnancies, which in turn have allowed veterinarians to take precautionary measures before the foal is due.
By monitoring the well-being of the fetus, mares which are having problem pregnancies can be assisted before the pregnancy ends with a premature delivery or a sick foal, or worse yet, the abortion of the foal. As a general rule, anything that makes you worry about the mare's health also should make you concerned with the health of the fetus.
According to Wendy Vaala, VMD, of Mid-Atlantic Equine Medical Center (Ringoes, N.J.), fetal well-being usually is checked during late pregnancy.
"Most mares I have looked at have been in the second half of pregnancy, particularly the last two months," said Vaala.
If the mare has experienced any health problems that possibly could be of concern, such as colic surgery, then monitoring the fetus should be undertaken, even if it is earlier in the pregnancy, said Vaala.
"Mares are generally a little more resilient in the second trimester," she said. "Even if the mares are sick, or have some other compromise, the pregnancy tends to hang in there."
She added a caveat that in the last couple of months of pregnancy, the risk of premature delivery seems to increase when you get a sick, toxic, or stressed mare.
"It's also more likely that you'll lose the pregnancy," she noted.
Which Mares to Watch
According to Vaala, there are three categories of pregnancies that warrant close monitoring for obvious reasons in order to avoid potential mishaps.
The first category consists of pregnant mares in late pregnancy which somehow are compromised themselves. A mare which falls into this category might have a medical problem such as severe pneumonia or diarrhea, EPM, or some type of trauma such as colic surgery. Whatever the cause, this late-stage pregnancy could be in jeopardy if the mare is not healthy. Therefore, not only do these mares warrant close monitoring by a veterinarian, but the practitioner needs to determine if the mare's illness or trauma has affected or will affect the fetus.
The second group of mares which should receive fetal monitoring is the group which has a history of reproductive problems. Whether the previous problem was twinning, premature delivery, or abortion, these mares are candidates for fetal evaluation during the last one to two months of pregnancy to check the status of the fetus and make sure it is progressing on schedule and within the normal parameters.
A third category of mares gives no signs of having trouble with their pregnancies, yet still give birth to premature or sick foals. It is not going to become routine for every mare on a farm to be monitored for fetal well-being, which makes it tricky to pick out members of this third group.
A mare in this category has shown no signs of an abnormal pregnancy, but if tests had been performed on her prior to foaling, an abnormality probably would have been revealed. The problem could arise from medical reasons from the mare herself that are not apparent, or it could just be "the nature of the beast." During her next pregnancy, this mare should be monitored to make sure everything is progressing normally in the pregnancy.
Monitoring The Fetus
Ultrasound is the primary tool used to monitor fetal well-being. In the later stages of pregnancy, ultrasound should be performed transabdominally with a sector scanner, which is a type of ultrasound probe. Vaala said most veterinarians will use a 2 1/2 megahertz (MHz) transducer. The megahertz of the transducer corresponds directly to the depth of penetration of the scan. In order to get enough depth penetration to go through the mare's abdominal wall and through the internal structures to visualize the fetus, a strong transducer is needed.
"You are going to be scanning from the ventral midline across the body wall into the abdomen and across the whole foal, which is a pretty large area," said Vaala. "You just clip (the hair off the mare's belly) and scan the whole ventral belly from in front of the udder up to just behind the sternum, then to either side of the midline, up along the side, almost parallel with the stifle."
The large area is clipped and prepared since the practitioner cannot predict how the fetus will be lying during the examination. Using coupling gel, the lubrication for the ultrasound scan, the scan is made through the mare's abdomen.
"Scanning this way is a non-invasive procedure in which the mare does not need to be sedated at all," said Vaala. "And with this technique, you can visualize most of the uterus and fetus. Plus, you see a lot of the placenta."
The fluids surrounding the fetus also are seen with the scan. In the horse, there are two fluid compartments surrounding the fetus--the amniotic fluid is in the sac right around the fetus, and the the allantoic fluid is in the sac around the amniotic compartment. These fetal fluids are composed of fetal urine, excretory products, and respiratory secretions.
The ultrasound scan provides a tremendous amount of information, but the drawback is that the equine fetus cannot all be seen in one view. Ultrasound provides information on fetal position, size and movement, fetal heart rate, placental thickness and integrity, and the volume and character of fetal fluids.
"It's frustrating for us (veterinarians) because we can only see bits and pieces of the foal because the foal is so large," said Vaala. "This is why you need to have the most powerful transducer, which is still not as good as we would like because it still does not allow us to see the whole foal."
Another method of monitoring fetal well-being is using an EKG machine, which is a simple, yet effective, way of measuring the heart rate of the fetus. There are portable units that a veterinarian can take out to the farm. The EKG leads are hooked up to the mare's abdomen to get a recording of the mother's heart rate and that of the fetus. This machine allows the veterinarian to document that the fetus is alive and that the heart rate is normal.
"Sometimes when the mare is sick or there is a problem with the pregnancy, a rectal exam is performed on the mare to check the fetus," said Vaala. "Sometimes the baby doesn't move when the uterus is touched. Is it dead? Asleep? Is it compromised somehow?"
With the EKG machine, at least death can be ruled out. And with a heartbeat within normal parameters, it means the risk of the fetus's being compromised is lower.
Work now is being conducted with the company that manufactures the Toitu monitor. This device currently is being used with pregnant women. It is strapped on women in labor rooms to monitor fetal activity and heart rate. This device has been used in horses for several years, according to Vaala, who said the data it provides is being looked at closely by researchers for its value in equine fetal monitoring.
"This data allows us to look at fetal movement and see what the heart rate does," Vaala added. "When there is a lot of fetal activity, the heart rate should begin to accelerate. If there is little movement, or no change in the heart rate with movement, then the pregnancy is suspect."
Drawing blood from the mare is yet another way to study the fetus. A simple blood test allows veterinarians to check hormone levels in the mare. The placenta and the fetus both play a role in producing the hormones that are detected in the pregnant mare's blood. If abnormal levels of progesterone or estrogen are found, it could be an indication that something is wrong with the pregnancy. These two hormones are analyzed late in the pregnancy, and they can be supplemented if the blood test reveals low levels.
Recently, a study at Rutgers University looked at relaxin levels. Vaala said, "This is a hormone produced by the equine placenta and, as its name implies, is released toward the end of pregnancy in order to relax connective tissue structures in the pelvis and cervix to allow the 125 pound fetus to emerge through the pelvis. However, we have only just been finding that abnormally low levels of relaxin have been associated with other forms of placental problems. This hormone offers some promise as another non-invasive approach used to monitor the fetus and placenta and what is going on with the pregnancy."
While the EKG, Toitu monitor, ultrasound, and blood tests are good because they are non-invasive ways to monitor the fetus's health, another way to determine fetal well-being is to perform amniocentesis, which is invasive. This is a technique which utilizes a six-to seven-inch needle inserted into the fetal fluid in order to draw a sample for analysis. This is a risky procedure for the veterinarian because in order to draw the fluid, the needle must be placed through the abdomen of the mare right in front of her mammary glands, which means the veterinarian is in a very good position to get kicked. It also is a risky procedure because it is done in to an alert, standing horse. The pocket of amniotic fluid is usually quite small and intimately associated with a moving fetus that can be poked by the tip of the needle.
"We don't do many of these because it's an invasive procedure," said Vaala.
When it is performed, this procedure usually is done to test for surfactant, which is the material that lines the alveoli, or air sacs of the lungs, and keeps them inflated once the foal is born and the first breaths are taken. Some premature foals do not have enough surfactant and their lungs collapse, which means they have to be placed on a ventilator to keep their lungs inflated until they start producing surfactant on their own. To aid the premature foal, surfactant can be administered to it at birth via the trachea.
"Amniotic fluid is analyzed because it is swallowed and ingested by the fetus," explained Vaala. "It is composed of digestive and respiratory secretions and is a reflection to some degree of lung maturation."
If infection is thought to be present, then amniotic and allantoic fluid can be cultured to see if infection is present and help determine the optimal antibiotic treatment program. Cultures also can be obtained from around the cervix which is the area where most placental infections begin.
What Are You Looking For?
The first thing that is monitored when the fetus is evaluated with ultrasound is its position. Ideally, the fetus is facing with its head near the mare's tail. Most foals are born in what is called the anterior presentation, where the nose, head, and front legs are the first body parts to appear out of the birth canal. An abnormal position (posterior presentation) would be when a foal has its rear end towards the mother's tail. After about nine months' gestation, the position of the fetus does not change very often, so how the foal is facing (i.e. anterior or posterior) indicates how the foal will be presented at birth.
A foal still can roll from one side to the other in late gestation, but it usually does not do a 180-degree turn around. The general position of the fetus is checked so that any fears of a breech birth can be put to rest, or if a posterior malposition is present, then at least the veterinarian is prepared. Even with the foal in an anterior presentation, a dystocia still can occur if a leg or head becomes caught or twisted during labor.
The next thing that is checked is the movement and tone of the fetus. According to Vaala, a healthy fetus moves quite frequently, several times at least in 10 minutes. While it makes the ultrasound scan tough, a moving fetus is a good sign. Also, if the fetus can move its extremities, then it has good muscle tone, according to Vaala.
The scan also will reveal if the fetus is breathing, because the fetus will breathe intermittently while in utero. This is a good sign. Heart rate also is checked in the fetus to see if it is within the normal readings for the stage of pregnancy. A good sign is an increased heart rate when the fetus moves. This is similar to an in utero stress test, said Vaala.
Amniotic fluid and allentoic fluid also are investigated.
"We get rough measurements of how much fluid is there by measuring pockets of fluid that we see around the foal in various areas, and we also look to see how clear the fluid is," explained Vaala. "Normally, it should be relatively clear. If it looks very flocculent, like you have floating particles in it, we call it echogenic (gives a lot of "echo" on the ultrasound, making it look more white than black on the screen). If it's very echogenic and has lots of particles swirling around in it, the problem may be an infection of the fluids and placenta, or from bleeding into the fetal fluids. This would not be a good sign and further evaluation of the pregnancy should be conducted, such as a vaginal exam, blood tests, and cultures."
The placenta also is studied in order to measure the placental thickness. When the placenta is compromised, whether due to infection or lack of blood supply, it becomes edematous (full of fluid) and thicker than normal, and it may separate from the wall of the uterus. The placenta is important to the fetus because here the nutrient exchange and oxygen exchange take place between dam and fetus. When the exchange is compromised and enough damage occurs to the placenta, problems such as asphyxia, early delivery, or abortion can be expected with the pregnancy.
Another important tool in monitoring fetal well-being is measurement of the fetal aorta--the large artery leaving the heart. The aortic diameter measurement closest to the heart gives a crude estimate of body size. This will alert the practitioner whether a large or small foal is expected.
What Can Go Wrong?
One of the first changes that occurs when the fetus is deprived of oxygen is that the heart rate slows down and the foal becomes hypoxic (low oxygen levels). More importantly, the heart rate will not accelerate if the fetus moves. This is an early indication that something is wrong.
Excessive amounts of fetal fluids are a condition commonly called "hydrops," where for various reasons too much fluid is produced by the dam and it becomes a life-threatening condition for the fetus and mare. In this situation, the mare becomes distressed by the accumulation of fluid, amounts that can reach up to a hundred liters of fetal fluid filling her abdomen and putting pressure on intestines, blood vessels, and other organs. She might show signs of colic and go off feed. These mares run the risk of dying of shock when they deliver because when the large amount of fluid is suddenly voided from their bodies, their blood pressure usually drops and they go into hypotensive shock. This condition is important to detect early so that there will be a controlled delivery with IV fluid and pressure support available for the mare when the foal is born. The foal often requires aggressive nursing care as well.
"The other condition we've only now recognized since we've been scanning mares is too little fetal fluids," said Vaala. "This can also be from a malfunctioning placenta and asphyxia, resulting in very little fluid around the fetus.This usually goes along with a compromised pregnancy and such babies are often starved of oxygen and probably smaller than normal size. Again, this should alert you that you will have a problem foaling."
One of the most important things when monitoring fetal health is that if you have a compromised fetus, then this is not a mare you want foaling out in a barn away from medical help. This mare should be referred to a large foaling facility or a veterinary clinic so she can foal under a veterinarian's supervision, which will increase the chances of a live foal and an unharmed mare. With these precautions, there will be assistance available for foaling or resuscitating or treating a foal. This extra care can determine whether you end up with a live foal and healthy mare or disaster. If there is a problem, then the foal is already in a setting where it can receive medical attention immediately, negating the need to ship a weak foal one or two hours to the nearest clinic after it is born, said Vaala.
About the Author
Tim Brockhoff was Staff Writer of The Horse:Your Guide to Equine Health Care from 1995 to 1999. His degree is in Agricultural Communications from the University of Kentucky, and his equine experience is with American Saddlebreds.
POLL: Colic Surgery