Marked variability in fetal heart rate is defined as:

Marked variability in fetal heart rate refers to the fluctuations in the heart rate that are greater than 25 beats per minute. This type of variability indicates a well-functioning autonomic nervous system and is often associated with a healthy fetus that is responding appropriately to stimuli. In clinical practice, marked variability is generally considered a reassuring sign of fetal well-being, suggesting that the fetus has good oxygenation and is not experiencing any distress.

Variability in fetal heart rate is categorized into different ranges: absent variability is characterized by a lack of fluctuations; minimal variability involves fluctuations of 0 to 5 bpm; moderate variability is defined as fluctuations between 6 and 25 bpm. Thus, the definition of marked variability being greater than 25 bpm highlights the threshold at which the fluctuations are significant enough to provide a positive indication of fetal health. This understanding is critical for healthcare professionals monitoring fetal status during labor and delivery.