Knowledge of the condition of the fetus in utero is entirely dependent on observation of the fetal heart tones. Methods of observation have varied as science has progressed and made available new instruments. Fetal heart tones were first heard with the unaided ear in 1818 by Major.1 It was not long before the newly invented stethoscope was also put into use. That the stethoscope itself was not a great advance is proved by the frequency with which one encounters direct auscultation, even at the present time. Later the head stethoscope was introduced, making it possible for the obstetrician to observe the heart tones without interfering with asepsis. However, at best it is an awkward, uncomfortable instrument.
With the improvement of electrical means of reproducing, transmitting and amplifying sound waves, it was inevitable that such methods should be applied to the study of adult and fetal heart sounds. In 1923
De Costa EJ. THE PHOTOSTETHOSCOPE: A DEVICE FOR VISIBLY DEMONSTRATING CARDIAC SOUNDS, ESPECIALLY FETAL HEART SOUNDS. JAMA. 1938;111(22):2008–2009. doi:10.1001/jama.1938.72790480006009c
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