The presence of fetal heart rate accelerations is one of the most important signs of well-being during labor. Accelerations are defined as short-term rises in the heart rate of at least 15 beats per minute, which last at least 15 seconds. In many cases, they last longer. Viewing the accelerations assures the doctor that the baby is not lacking oxygen or accumulating acid, which results from lack of oxygen. Most fetuses have spontaneous accelerations at various points throughout the labor and delivery process. If a doctor is concerned about the well-being of a baby and does not see accelerations, she may attempt to induce accelerations either by gently rocking the mother's abdomen, pressing on the baby's head through the cervix with a finger, or administering a short burst of sound (vibro acoustic stimulation). If these techniques trigger accelerations, the doctor or nurse is reassured the baby is doing fine.
Decelerations are temporary drops in the fetal heart rate. There are three basic types of decelerations: early decelerations, late decelerations, and variable decelerations.
Early decelerations are seen when the baby's head is compressed. This often happens during the later stages of labor as the baby is descending through the birth canal. They may also occur during early labor if the baby is premature or in a breech position, causing its head to be squeezed by the uterus during contractions. Early decelerations have absolutely no clinical significance and are not harmful.
Unlike early decelerations, which begin to dip early during the uterine contraction or even before the contraction is visible, late decelerations do not begin until the peak of a contraction or thereafter. They are smooth, shallow dips in heart rate that mirror the shape of the contraction that is causing them.
Late decelerations are among the most worrisome form of heart rate patterns because they usually signify a reduced oxygen supply to the baby. When delivery is near and there are other reassuring features of the heart rate tracing (such as accelerations), it is often permissible to observe a fetus with late decelerations carefully and not intervene with a cesarean section. However, if worrisome features of heart rate tracing are present, such as a fast heart rate (tachycardia), reduced variability, and an absence of accelerations, a rapid delivery might be called for, since prolonged exposure to the type of contractions causing late decelerations may be harmful to the baby. This might be done by cesarean section or by assisting delivery with a vacuum or forceps. Most babies with late decelerations progress normally, especially if the other worrisome features described above are not present. In one study, 99.8% of babies who had both late decelerations and reduced variability were normal and it made no difference in the remaining 0.2% whether the baby was delivered vaginally or by cesarean section. Most medical scientists believe that many abnormal fetal heart rate patterns are simply indicators of a baby that is not tolerating labor because of abnormal development, rather than indicators of harm caused by decelerations.
The mechanism that produces late decelerations is fascinating. Sea mammals, such as seals or whales, often dive thousands of feet below the surface of the ocean in search of food. Because these mammals breathe air, they must hold their breath during these dives and the oxygen content of their bodies invariably falls during these prolonged dives. Sea mammals can adapt to these periods of low oxygen by slowing their heart rate, thus utilizing less oxygen. This reflex in humans is known as the diving seal reflex. Before we are born, we also have the ability to react with a diving seal reflex, lowering our heart rates when oxygen supply is reduced. During all normal uterine contractions, the oxygen supply to the baby is temporarily reduced or even stopped because the blood vessels supplying the baby's oxygen go through the contracting uterine muscle. These reductions in oxygen supply can prompt some babies to utilize the protective diving seal reflex and temporarily slow their heart rates until oxygen supply is restored. As adults, we tend to lose this reflex. However, trained divers can regain the diving seal reflex.
Variable decelerations do not look like late or early decelerations. They are generally irregular, often jagged dips in the fetal heart rate that look more dramatic than late decelerations. Variable decelerations are caused when the umbilical cord of the baby is temporarily compressed. This happens in virtually all labors, and multiple variable decelerations can be found during the course of nearly all labors. As a rule, variable decelerations are not worrisome. However, the baby does depend upon steady blood flow through the umbilical cord to receive oxygen and other important nutrients. If variable decelerations are prolonged or repetitive, they can signify a reduction of blood flow, which is harmful to the baby. Doctors judge whether variable decelerations are worrisome or not primarily depending on the other features of the heart rate tracing, and how close to delivery they judge the woman to be. For example, severe variable decelerations with no variability in early labor would be an indication for cesarean section; the same decelerations seen with good variability and accelerations close to delivery would not require cesarean section.