What is a defining feature of Category III fetal heart tones (FHT)?

A defining feature of Category III fetal heart tones (FHT) is the presence of bradycardia without variability. Category III represents abnormal fetal heart rate patterns that are associated with an increased risk of adverse outcomes, including fetal distress.

In Category III, the hallmark is a persistent heart rate below 110 beats per minute (bradycardia) that lacks variability, meaning there is little to no fluctuation in the heart rate. This lack of variability can indicate significant fetal compromise, as well as an inability of the fetal autonomic nervous system to generate a normal heart rate response.

When considering other options: while a sinusoidal pattern and the absence of variability are indeed characteristics of abnormal heart rate patterns, the specific defining feature of Category III is the combination of bradycardia and the absence of variability. Moderate baseline variability and the presence of accelerations typically indicate better fetal oxygenation and are categorized as either Category I or II. Hence, they do not fit within the definition of Category III patterns.