What is the most common cause of sustained tachycardia in a term fetus?

Sustained tachycardia in a term fetus is most commonly caused by maternal or fetal infection. Infections can lead to an elevated heart rate in the fetus as a response to elevated temperature and inflammation, often caused by conditions such as chorioamnionitis or maternal fever. These infectious processes can trigger the fetal immune response, resulting in tachycardia as the fetus attempts to cope with the stress of the infection.

While maternal hypertension, fetal heart abnormalities, and placental insufficiency can also be associated with changes in fetal heart rate, they are less frequently the primary causes of sustained tachycardia. Maternal hypertension primarily leads to complications such as fetal distress or compromised blood flow, while fetal heart abnormalities directly involve structural issues with the heart that may not manifest as sustained tachycardia. Placental insufficiency tends to result in chronic signs of distress rather than acute tachycardia alone. Therefore, the presence of an infection is a more prevalent and direct cause of sustained tachycardia in the term fetus.