Ultrasound is a method of assessing the fetus using low-frequency sound waves to reflect off fetal tissue. The ultrasound transducer produces ultrasound waves, which bounce off tissue at different speeds depending on its density. Most commercial ultrasound equipment emits energy that is much lower than the determined maximum safety standard. There are no known reports of fetal damage from conventional diagnostic ultrasound.
There is no uniform agreement as to when ultra-sound should be performed during pregnancy. Nevertheless, ultrasound has become the predominant method for determining fetal age, assessing fetal anatomy, and monitoring fetal growth. The American Institute of Ultrasound in Medicine recommends that ultrasound be used in the first trimester to determine fetal age, number, and viability (via visualization of fetal heart activity). In the second and third trimesters, the fetus can be scanned for anatomic abnormalities, fetal growth, amniotic fluid volume, and placental location.
Creasy, Robert K., and Robert Resnik. Maternal-Fetal Medicine. Philadelphia: Saunders, 1999.
Gabbe, Steven, Jennifer R. Neibyl, and Joseph L. Simpson. Obstetrics: Normal and Problem Pregnancies. New York: Churchill Livingstone, 1997.