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Basic principles and safety of diagnostic ultrasound in obstetrics and gynecology

Thomas D Shipp, MD, RDMS
Section Editor
Deborah Levine, MD
Deputy Editor
Vanessa A Barss, MD, FACOG


The term "ultrasound" refers to sound waves of a frequency greater than that which the human ear can appreciate, namely frequencies greater than 20,000 cycles per second. For diagnostic ultrasound imaging in obstetrics and gynecology, frequencies of 2 to 12 million cycles per second are used. Ultrasound imaging has been used for medical purposes for several decades and is safe when properly performed [1].

Understanding the physical principles underlying ultrasound technology can help the sonographer optimize image quality, and thus improve diagnostic capabilities. This information is also vital for maintaining the safety of this technology for the woman and, during obstetrical examinations, her fetus.


Sound waves are a type of mechanical vibration. They are described in terms of their frequency, which is the number of repetitions (ie, cycles) per second. The unit for measuring frequency is the Hertz (Hz). Other characteristics of sound waves are wavelength, the distance between excitations; the amplitude of excitation, measured in decibels (dB); and the period, the time it takes for one cycle to occur, measured in seconds.

The speed with which an acoustic wave moves through a medium is dependent upon the density and resistance of the medium. Media that are dense will transmit a mechanical wave with greater speed than those that are less dense. As an example, the acoustic speed of a mechanical wave through air is 331 meters/second; through water, it is 1495 meters/second; through soft tissue, it is 1540 meters/second; and through bone, it is 4080 meters/second [2].

Frequency and wavelength are mathematically related to the velocity of the ultrasound beam within the tissue as indicated by the following equation:

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Literature review current through: Nov 2017. | This topic last updated: Dec 01, 2017.
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  1. Phillips RA, Stratmeyer ME, Harris GR. Safety and U.S. Regulatory considerations in the nonclinical use of medical ultrasound devices. Ultrasound Med Biol 2010; 36:1224.
  2. Ziskin MC. Basic physics of ultrasound. In: The Principles and Practice of Ultrasonography in Obstetrics and Gynecology, 4th ed, Fleischer AC, Romero R, Manning FA, et al (Eds), Appleton & Lange, Norwalk, CT 1991.
  3. Roberts WE. Practical and financial considerations that affect selection and purchase of ultrasound equipment. Obstet Gynecol Clin North Am 1998; 25:663.
  4. Brant WE. Ultrasound basics: Getting started. In: Ultrasound: The Core Curriculum, Lippincott Williams & Wilkins, New York 2001.
  5. Thornton KL. Principles of ultrasound. J Reprod Med 1992; 37:27.
  6. Drose JS. The mechanics of ultrasound. In: Diagnostic Ultrasound in Clinical Obstetrics and Gynecology, Thompson HE, Bernstine RL (Eds), Wiley & Sons, New York 1978.
  7. FitzGerald DE, Drumm JE. Non-invasive measurement of human fetal circulation using ultrasound: a new method. Br Med J 1977; 2:1450.
  8. Gill RW. Pulsed Doppler with B-mode imaging for quantitative blood flow measurement. Ultrasound Med Biol 1979; 5:223.
  9. Eik-Nes SH, Brubakk AO, et al. Measurement of human fetal blood flow. Br Med J 1980; 28:283.
  10. Maulik D, Nanda NC, Saini VD. Fetal Doppler echocardiography: methods and characterization of normal and abnormal hemodynamics. Am J Cardiol 1984; 53:572.
  11. Arbeille P, Tranquart F, Body G, et al. Evolution de la circulation arterielle ombilicale et cerebrale du foctus au cours de la grossesse. Progres en Neonatologie 1986; 6:30.
  12. Baran JM, Webster JG. Design of low-cost portable ultrasound systems: review. Conf Proc IEEE Eng Med Biol Soc 2009; 2009:792.
  13. Harris RD, Marks WM. Compact ultrasound for improving maternal and perinatal care in low-resource settings: review of the potential benefits, implementation challenges, and public health issues. J Ultrasound Med 2009; 28:1067.
  14. Spencer JK, Adler RS. Utility of portable ultrasound in a community in Ghana. J Ultrasound Med 2008; 27:1735.
  15. Perk G, Molisse T, Remolina A, et al. Laptop-sized echocardiography machine versus full-sized top-of-the-line machine: a comparative study. J Am Soc Echocardiogr 2007; 20:281.
  16. Merritt CR, Kremkau FW, Hobbins JC. Diagnostic ultrasound: bioeffects and safety. Ultrasound Obstet Gynecol 1992; 2:366.
  17. American College of Obstetricians and Gynecologists. Committee opinion no. 180: New ultrasound output display standard. Washington, DC, 1996.
  18. Barnett SB, Ter Haar GR, Ziskin MC, et al. International recommendations and guidelines for the safe use of diagnostic ultrasound in medicine. Ultrasound Med Biol 2000; 26:355.
  19. Standard for Real-time Display of Thermal and Mechanical Acoustic Output Indices on Diagnostic Ultrasound Equipment, Revision 1, 1998.
  20. Canadian Guidelines for the Safe Use of Diagnostic Ultrasound. www.hc-sc.gc.ca/hecs-sesc/ccrpb/publication/01hecs255/chapter2.htm (Accessed on March 07, 2005).
  21. Sheiner E, Abramowicz JS. A symposium on obstetrical ultrasound: is all this safe for the fetus? Clin Obstet Gynecol 2012; 55:188.
  22. Calvert J, Duck F, Clift S, Azaime H. Surface heating by transvaginal transducers. Ultrasound Obstet Gynecol 2007; 29:427.
  23. Ang ES Jr, Gluncic V, Duque A, et al. Prenatal exposure to ultrasound waves impacts neuronal migration in mice. Proc Natl Acad Sci U S A 2006; 103:12903.
  24. Whitworth M, Bricker L, Mullan C. Ultrasound for fetal assessment in early pregnancy. Cochrane Database Syst Rev 2015; :CD007058.
  25. American Institute of Ultrasound in Medicine. AIUM practice guideline for the performance of obstetric ultrasound examinations. J Ultrasound Med 2013; 32:1083.
  26. Callen PW. The obstetric ultrasound examination. In: Ultrasonography in Obstetrics and Gynecology, 4th ed, Callen PW (Ed), WB Saunders Company, New York 2000.
  27. Reece EA, Assimakopoulos E, Zheng XZ, et al. The safety of obstetric ultrasonography: concern for the fetus. Obstet Gynecol 1990; 76:139.
  28. Miller MW, Brayman AA, Abramowicz JS. Obstetric ultrasonography: a biophysical consideration of patient safety--the "rules" have changed. Am J Obstet Gynecol 1998; 179:241.
  29. Abramowicz JS, Kossoff G, Marsal K, et al. Safety Statement, 2000 (reconfirmed 2003). International Society of Ultrasound in Obstetrics and Gynecology (ISUOG). Ultrasound Obstet Gynecol 2003; 21:100.
  30. Torloni MR, Vedmedovska N, Merialdi M, et al. Safety of ultrasonography in pregnancy: WHO systematic review of the literature and meta-analysis. Ultrasound Obstet Gynecol 2009; 33:599.
  31. Salvesen KÅ. Ultrasound in pregnancy and non-right handedness: meta-analysis of randomized trials. Ultrasound Obstet Gynecol 2011; 38:267.
  32. Salvesen KA, Lees C. Ultrasound is not unsound, but safety is an issue. Ultrasound Obstet Gynecol 2009; 33:502.
  33. Abramowicz JS. Ultrasound and autism: association, link, or coincidence? J Ultrasound Med 2012; 31:1261.
  34. American Institute of Ultrasound in Medicine, Official Statement, Prudent Use and Clinical Safety, American Institute of Ultrasound in Medicine, March 2007.
  35. American Institute of Ultrasound in Medicine, Official Statement, Conclusions Regarding Epidemiology for Obstetric Ultrasound, American Institute of Ultrasound in Medicine, June 2005.
  36. Medical Ultrasound Safety, American Institute of Ultrasound in Medicine, Second Edition, 2009.
  37. Center for Devices and Radiological Health. Information for Manufacturers Seeking Marketing Clearance of Diagnostic Ultrasound Systems and Transducers. www.fda.gov/cdrh/ode/ulstran.pdf (Accessed on March 07, 2005).
  38. Nemescu D, Berescu A, Onofriescu M, et al. Safety Indices during Fetal Echocardiography at the Time of First-Trimester Scan Are Machine Dependent. PLoS One 2015; 10:e0127570.
  39. Nemescu D, Berescu A. Acoustic output measured by thermal and mechanical indices during fetal echocardiography at the time of the first trimester scan. Ultrasound Med Biol 2015; 41:35.
  40. Sheiner E, Shoham-Vardi I, Abramowicz JS. What do clinical users know regarding safety of ultrasound during pregnancy? J Ultrasound Med 2007; 26:319.
  41. Bhide A, Acharya G, Bilardo CM, et al. ISUOG practice guidelines: use of Doppler ultrasonography in obstetrics. Ultrasound Obstet Gynecol 2013; 41:233.
  42. Salvesen K, Lees C, Abramowicz J, et al. ISUOG statement on the safe use of Doppler in the 11 to 13 +6-week fetal ultrasound examination. Ultrasound Obstet Gynecol 2011; 37:628.
  43. Bly S, Van den Hof MC, Diagnostic Imaging Committee, Society of Obstetricians and Gynaecologists of Canada. Obstetric ultrasound biological effects and safety. J Obstet Gynaecol Can 2005; 27:572.
  44. Abramowicz JS. Fetal Doppler: how to keep it safe? Clin Obstet Gynecol 2010; 53:842.
  45. Bioeffects and Safety Committee, Salvesen K, Lees C, et al. ISUOG-WFUMB statement on the non-medical use of ultrasound, 2011. Ultrasound Obstet Gynecol 2011; 38:608.
  46. Committee on Practice Bulletins—Obstetrics and the American Institute of Ultrasound in Medicine. Practice Bulletin No. 175: Ultrasound in Pregnancy. Obstet Gynecol 2016; 128:e241.