Doppler ultrasound monitors are commonly used to measure the fetal heart rate (FHR) in routine screenings. They are less intrusive than fetal electrocardiography (FECG), but are more prone to noise and thus seen as less accurate devices. Recent developments resulted in beat-to-beat heart rate algorithms for Doppler shift monitors that claimed to approach the accuracy of that obtained from direct FECG. We evaluate one such algorithm, along with a noise tolerant improvement using a mixture of synthetic and real fetal heart data in order to quantify their accuracy and robustness in the presence of the typical noise in signals from an ultrasound probe. The findings showed that our proposed approach achieved higher accuracy and better robustness compared to the beat-to-beat fetal heart detection approach.