Obstetrical ultrasound is fundamental to prenatal care. It is used to confirm gestational age and viability; to detect and characterize abnormalities of the fetus, amnionic fluid, and placenta; and to assist with diagnostic and therapeutic procedures.
Ultrasound practice continues to evolve. The number of components included in the second-trimester standard and detailed fetal anatomical surveys has expanded. With improved image resolution, fetal abnormalities are increasingly identified in the late first trimester. This has prompted the requirement for a limited anatomical survey during the standard first-trimester examination and has led to the development of a new detailed first-trimester examination. Detailed placental evaluation is a new specialized examination to aid detection and characterization of placenta accreta spectrum.
Across the United States, pregnant women receive ultrasound examinations in various practice settings staffed by obstetrician–gynecologists, maternal–fetal medicine specialists, and radiologists. Ideally, examinations are performed by registered diagnostic medical sonographers or physicians with certification in their area(s) of practice and in units accredited by the American Institute of Ultrasound in Medicine (AIUM) or American College of Radiology. Components of accreditation include evidence of physician training, sonographer credentialing, continuing medical education, and protocols and procedures to ensure proper and safe ultrasound practice. One important component is independent review of submitted images.
To standardize ultrasound education for residents in obstetrics and gynecology and fellows in maternal-fetal medicine, the American College of Obstetricians and Gynecologists, the Society for Maternal-Fetal Medicine, and the AIUM have developed consensus documents (Abuhamad, 2018; Benacerraf, 2018). A focus of training—and of ultrasound practice—is standardization of an ultrasound curriculum and provision of competency assessment tools for image acquisition.
The 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. Ultrasound transducers contain groups of piezoelectric crystals that convert electrical energy into sound waves and convert returning sound waves back into electrical energy. The sound waves are emitted in synchronized pulses. As these pulses pass through tissue layers, dense tissue such as bone produces high-velocity reflected waves. With routine gray-scale imaging, which is also known as brightness-mode (B-mode), these reflected waves are displayed as bright echoes on the screen. Conversely, fluid generates few reflected waves and appears dark. 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 (Hz [cycles per second]). Transducers use wide-bandwidth technology to perform within a range of frequencies. Higher-frequency transducers yield better image resolution, whereas lower frequencies penetrate tissue more effectively. In early pregnancy, a 5- to 12-megahertz (MHz) transvaginal transducer usually provides excellent resolution, because the early fetus lies close to the transducer. In the first and second trimesters, a 4- to 6-MHz transabdominal transducer is similarly sufficiently close to the ...