External Fetal Monitors – A Popular Use of Ultrasound in Obstetrics

Real-time data on a pregnant mother and her unborn child during the course of labor was a pressing need in obstetrical science. Ultrasound came to the rescue with the development of external fetal monitors. The all-encompassing term for this type of monitoring is cardiotocography, or CTG. This refers to the fact that not one, but two pieces of data are being collected. In simple terms, the CTG is recording the fetal heartbeat and the rate of uterine contractions. These factors, when taken together, are a good indicator of how the fetus is handling the stress of labor.

There are two ultrasound transducers used in fetal monitoring, one for the fetal heart rate and the other for the contractions themselves. External monitoring is accomplished by strapping the transducing sensors to the abdomen of the pregnant mother. The heart sensor is based on Doppler methodology. The contraction transducer is a pressure sensor and measures the tension of the abdominal wall which is an indirect indicator of intrauterine pressure.

CTG must consider several factors, namely uterine contractions and all four features of fetal heart rate:

*the baseline heart rate,
*variability or changes over time,
*accelerations over time, and
*decelerations over time.

Before interpretation can begin, risk factors must be defined. They will influence the decision-making process. For example, the decision to proceed to a surgical intervention or cesarean section might be made more quickly if the patient is at higher risk for an adverse outcome.

Historically, this suite of data has been classified as “reassuring” or “non-reassuring.” Deemed not descriptive enough, these two classifications have been supplanted by these new categories.

*Category I (normal) – the tracing of all findings are indicative of normal status and labor can be allowed to proceed using standard procedures.
*Category II (indeterminate) – the tracing of all findings are not indicative of normal status but do not rise to the level that requires immediate intervention. Continued monitoring is indicated and more re-evaluations are necessary.
*Category III (abnormal) – some part of the findings indicate an abnormal status such as fetal distress that requires prompt evaluation and management, inducing but not limited to direct intervention such as a cesarean section.