The primary electrical impulses stimulating the fetal heart to contract originate in the:

The sinoatrial (SA) node is the primary pacemaker of the heart, both in adults and in fetuses. It generates electrical impulses that initiate each heartbeat, setting the rhythm for the heart's contractions. In the fetus, the SA node is particularly crucial as it establishes the heart rate and is responsible for the coordinated contractility of the heart muscle.

The other options listed contribute to the conduction system of the heart, but they do not initiate heart contractions. The atrioventricular (AV) node serves as a relay point between the atria and ventricles but does not generate impulses. The bundle of His is responsible for conducting impulses from the AV node down through the ventricles but does not initiate the heart's rhythm. Purkinje fibers distribute the electrical impulse throughout the ventricles, ensuring coordinated contraction but again, do not generate impulses. Therefore, the SA node is the correct answer as it is the primary source of electrical impulses stimulating fetal heart contractions.