Comprehensive and Detailed Explanation From NCC-Aligned Sources:

Because the fetal myocardium is immature, it has:

Limited ability to increase stroke volume

Limited ability to increase contractility

Therefore, fetal cardiac output (CO) is almost entirely dependent on heart rate.

NCC and AWHONN physiology describe:

CO = stroke volume × heart rate

In the fetus, stroke volume is relatively fixed

Therefore, changes in HR directly affect cardiac output

Tachycardia → increases CO

Bradycardia → decreases CO → decreased perfusion and oxygen delivery

Why the other options are incorrect:

A. Activity does not fundamentally determine CO.

B. Baroreceptors regulate HR reflexively but are not the primary determinant of cardiac output.

Correct answer: C. Heart rate