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How the Fetal Monitor Works

Using an electronic fetal monitor is the best method of monitoring your baby during labor. It is the one piece of equipment that is routinely used in all labors, both normal and abnormal.

The fetal monitor looks quite impressive, but it's just a more sophisticated version of the monitor that your practitioner used to check the baby's heart rate at each prenatal visit. It operates on the same principle, using ultrasound waves to detect the movement of the fetal heart, and translating the changes in the waves to sound. In addition, the monitor can create a visual record of the heart rate because it contains a computer that instantaneously calculates the heart rate and displays the number. A pen continuously records the changing heart rate, producing a permanent graphic record. A second pen records the contraction pattern below the heart rate on the monitor paper. Monitoring contractions is very important because the heart rate pattern is almost impossible to interpret, unless you know when the contractions are occurring.

What do we learn from fetal monitoring? Monitoring the fetal heart rate can give valuable information about the baby's well-being, or health, in the last trimester of pregnancy. Also, fetal heart rate monitoring can be used to assess how well the fetus is tolerating the stresses of labor.

The non-stress test is an important method used to assess the condition of the fetus before labor begins. The name refers to the fact that when the test is administered, the fetus is not being subjected to the stress of contractions. A healthy baby, receiving an adequate supply of oxygen, will demonstrate a heart rate in the normal range (120-160 beats per minute). Also, a healthy baby is expected to move intermittently. This will be reflected by a rising heart rate that gradually returns to normal when the baby stops moving. Most babies who are receiving an adequate supply of oxygen will experience at least two accelerations (increases in heart rate) of at least 15 beats per minute above the baseline heart rate during a 20-minute period. These accelerations are usually associated with fetal movement. A healthy baby who is receiving adequate oxygen will move frequently. An oxygen-deprived fetus will move sluggishly or not at all.

There should not be any decelerations (dips in the heart rate) on a normal non-stress test. A non-stress test is called reactive if the results are normal, an excellent prognostic sign. If the heart rate is not reactive, the fetus may be okay, but further testing is clearly indicated.

The process of monitoring the fetal heart rate in the presence of contractions is called a contraction stress test, although the test is done exactly the same way as the non-stress test. When interpreting a contraction stress test, two questions must be asked. Is the fetal heart rate reactive? And, how does the heart rate respond to the contractions? A fetus that was fine before labor started may have trouble once contractions begin to occur regularly.

The baby who is receiving adequate oxygen across the placenta should not show any change in heart rate in response to the contractions. However, sometimes the fetal heart rate will decelerate. Not all decelerations are ominous. The significance of decelerations is determined by their relationship to the contractions-late decelerations cause the greatest concern. Late decelerations occur as each contraction is ending and persist after the contraction has stopped. Repetitive late decelerations are a sign of utero-placental insufficiency. This means the placenta is unable to supply enough oxygen between contractions to allow the fetus to "hold its breath" during contractions. Late decelerations warrant further testing if delivery is not imminent, and indicate a need for supplemental oxygen and other maneuvers that may increase the amount of oxygen available to the fetus. If late decelerations persist despite all efforts at treatment, emergency delivery may be needed.

Bradycardia, or slow heart rate, is another sign of significant fetal compromise.

If the fetal heart rate drops below 100 for more than one minute, the dip is no longer called a deceleration; it has now become a bradycardia. A drop in heart rate lasting several minutes or more is, of course, cause for serious concern. If the usual treatment measures fail to correct the problem, emergency delivery is required. Sometimes a lack of oxygen causes bradycardia. Other less common causes of bradycardia include compression of the umbilical cord, or even a true knot in the cord.