A great hallmark in obstetrical history began in the second half of the 20th century with the ability to image the pregnant uterus and its contents. Beginning with sonographic imaging and continuing through computed tomographic and magnetic resonance imaging, obstetrical practice was revolutionized and gave birth to the specialty of fetal medicine. Today’s practitioner can hardly imagine obstetrical care without these technical advances, which have become commonplace and regarded almost as a sixth sense.
Sonography in prenatal care includes first- and second-trimester fetal anatomic evaluation and specialized studies performed to characterize abnormalities. With improvements in resolution and image display, anomalies are increasingly diagnosed in the first trimester. Applications for three-dimensional sonography and Doppler continue to expand. A sonographic examination performed with the exacting recommended standards of the American Institute of Ultrasound in Medicine (2013a) offers vital information regarding fetal anatomy, physiology, growth, and well-being. Indeed, a National Institute of Child Health and Human Development (NICHD) workshop concluded that “every fetus deserves to have a physical examination” (Reddy, 2008).
The real-time image on the ultrasound screen is produced by sound waves that are reflected back from fluid and tissue interfaces of the fetus, amnionic fluid, and placenta. Sector array transducers used in obstetrics contain groups of piezoelectric crystals working simultaneously in arrays. These crystals convert electrical energy into sound waves, which are emitted in synchronized pulses. Sound waves pass through tissue layers and are reflected back to the transducer when they encounter an interface between tissues of different densities. Dense tissue such as bone produces high-velocity reflected waves, which are displayed as bright echoes on the screen. Conversely, fluid generates few reflected waves and appears dark—or anechoic. Digital images generated at 50 to more than 100 frames per second undergo postprocessing that yields the appearance of real-time imaging.
Ultrasound refers to sound waves traveling at a frequency above 20,000 hertz (cycles per second). Higher-frequency transducers yield better image resolution, whereas lower frequencies penetrate tissue more effectively. Transducers use wide-bandwidth technology to perform over a range of frequencies. In the second trimester, a 4- to 6-megahertz abdominal transducer is often in close enough proximity to the fetus to provide precise images. By the third trimester, however, a lower frequency 2- to 5-megahertz transducer may be needed for penetration, but can lead to compromised resolution. This explains why resolution is often poor when imaging obese patients and why low-frequency transducers are needed to reach the fetus through maternal tissues. In early pregnancy, a 5- to 10-megahertz vaginal transducer may provide excellent resolution, because the early fetus is close to the transducer.
Sonography should be performed only for a valid medical indication, using the lowest possible exposure setting to gain necessary information—the ALARA principle—as low as reasonably achievable. It should be ...