Bleeding in the third trimester is always a potentially hazardous complication of pregnancy, and its cause must be determined before the obstetrician can decide on proper care of the patient and the management of labor. At times, bleeding is caused by placenta praevia; therefore it is important to diagnose or rule out this complication. In the past several years, localization of the placenta has become increasingly important when amniocentesis or intrauterine transfusions are to be performed, so that the placenta will not be penetrated during these procedures. This requires accurate localization in three dimensions—anterior to posterior, right to left, and inferior to superior. This type of localization may be required as early as the middle of the second trimester. The following technics have been or are being used for placental localization: (a) soft-tissue x-ray technics (1); (b) cystography (2, 3); (c) angiography (4–6); (d) intravenous placentography (7); (e) isotope localization with radioactive sodium (8), iodine 131 (6, 9–11), or chromium 56(12–14), (f) amniography (15); and (g) thermography (16–18). All of these methods can be used for localization of the placenta with varying degrees of accuracy. With the exception of thermography, all have some element of risk because they involve exposure to radiation or radioactive materials or dangerous injection routes. Thermography, the use of infrared sensing and recording, eliminates the need for ionizing material and is accurate for vertical localization but not as accurate for anterior or posterior, i.e., horizontal localization. It is a comparatively new technic and needs additional use before its value can be fully assessed. Accuracy, safety, and simplicity are, of course, the primary considerations, and recently ultrasound has been proposed as fulfilling these criteria (19, 20). Ultrasound does not expose the mother or fetus to ionizing energy and appears to be easier to use than thermography. We therefore decided to investigate the application of ultrasound technics to localization of the placenta. Principles and Equipment The instrument used in this study3 is based on the Doppler principle, i.e., sound waves are altered in frequency when they are reflected from a moving target. Ultrasound waves are transmitted from a transducer, which contains a transmitting and receiving crystal, and pass through the skin and vessels being examined. These ultrasound waves have a frequency of 2 megacycles/sec. with an intensity of approximately 10 milli-watts/cm2. The reflected waves, changed in frequency, are received by the transducer, amplified, filtered, and converted into audible signals. Motion of the fetal heart and blood flow in the umbilical cord, in the placenta, and in fetal and maternal blood vessels can be detected by this instrument. Each has a distinctive characteristic sound, and with practice it is easy to distingush the different sources.