How To Use A Doppler: Finding the heartbeat in early pregnancy (8+ weeks) - YouTube
Table 2 from Uterine Artery Doppler Flow Indices in Pregnant Women During the 11 Weeks + 0 Days and 13 Weeks + 6 Days Gestational Ages: a Study of 168 Patients. | Semantic Scholar
BabyHeart x Hi-Bebe Fetal Doppler (Limited Edition)