What is Fetal Heart Rate Monitoring?
Fetal monitoring is defined as watching the baby's heart rate for indicators of stress. It is a tool to help doctors diagnose and treat potential problems during pregnancy, labor and after birth. There are two methods of fetal heart rate monitoring: auscultation and electronic fetal monitoring.
An electronic fetal monitor, or EFM, is a device for observing and recording the heart rate of a fetus, and for keeping track of the frequency, length, and strength of contractions during labor. The machine used to perform the monitoring is called a cardiotocograph. It continuously prints out a record of both the fetal heart rate, and the duration and frequency of the contractions, so that deviations from normal patterns can be identified. Electronic fetal monitoring can be external or internal.
External monitoring utilizes ultrasound waves to monitor fetal health. This method is performed using a hand-held Doppler ultrasound probe to listen to and count the fetal heart rate during a contraction to identify fetal response. It may also be performed using an external transducer, which is placed on the maternal abdomen and held in place by an elastic belt or girdle. The transducer uses Doppler ultrasound to detect fetal heart motion and is connected to a fetal heart rate monitor. The monitor calculates and records the fetal heart rate on a continuous strip of paper.
If it becomes difficult to detect the fetal heart rate with the external monitor or if there are subtle signs of a developing problem, an internal monitor may be used. Internal monitoring is monitoring with an electrode attached to the baby's head to record heart tones, and a pressure catheter to record contractions. A spiral wire, called the fetal scalp electrode, is placed just beneath the skin of the baby's scalp. The fetal scalp electrode then transmits direct information about the fetus' heart rate through the wire to the fetal monitor that prints out this information. Because the internal fetal monitor is attached directly to the baby, the fetal heart rate signal is sometimes much clearer and more consistent than with an external monitoring device.
One benefit of electronic fetal monitoring is to detect early fetal distress resulting from fetal hypoxia. The average fetal heart rate is between 110 and 160 beats per minute. Fetal hypoxia occurs as a result of deprivation of oxygen to the fetus, which has been implicated as a cause of cerebral palsy.
Although detection of fetal distress is one benefit of fetal monitoring, there are also risks. The major risk associated with electronic fetal heart rate monitoring is a false positive test that may result in unnecessary surgical intervention such as cesarean section or a delivery by forceps or vacuum. A false positive result is when the information is interpreted as indicating distress but there is no fetal distress. A false negative may result when the information from monitoring indicates that the baby is healthy but the baby is born with problems. This type of error is less common.
Other minor risks of fetal monitoring may arise when internal monitoring is necessary. Internal monitoring requires a small wire to be placed beneath the skin of the baby's scalp. This results in a small break in the skin that usually heals without a problem, but in rare cases may result in bleeding or a skin infection. This infection may cause fever during labor.
This year the American College of Obstetricians and Gynecologists (ACOG) revised its guidelines on fetal heart rate monitoring, which were published in the July issue of Obstetrics & Gynecology.