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KEY QUESTIONS
How can ultrasound be used to assess fetal growth and well-being?
How can ultrasound be used to evaluate preterm labor?
How can ultrasound be used in the evaluation of third-trimester bleeding?
How can ultrasound be used in the assessment of postpartum bleeding?
How can ultrasound be used to detect lethal anomalies and for intrapartum management of these anomalies?
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CASE 65-1
A 24-y.o. homeless, obviously gravid patient is brought in to L&D by ambulance, complaining of abdominal pain. On your initial evaluation, she states that she has had no prenatal care and does not remember her last menstrual period. She has a history of one vaginal delivery “a few weeks early” 3 years ago. The FHR tracing reveals a baseline of 150 bpm, with moderate variability and no accelerations or decelerations. She is contracting irregularly but painfully every 5–10 minutes.
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The use of ultrasound in obstetrics has become a vital component in assessing the well-being of the fetus and fetal growth as well as in quickly evaluating common obstetric complications. Familiarity with the basic principles and several high-yield applications of obstetric ultrasound is an important component of the well-prepared obstetric and gynecologic (OB/GYN) hospitalist’s skill set, allowing a more complete, rapid assessment of patients in acute situations.
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As with all modalities of imaging in the pregnant patient, the use of ultrasound abides by the ALARA principle (which stands for “as low as reasonably achievable”).1 The American College of Obstetrics and Gynecology (ACOG) and the American Institute of Ultrasound in Medicine (AIUM) agree that the use of ultrasound during pregnancy is not associated with risk to either the fetus or the pregnant patient; however, ultrasound should be performed only by those trained to recognize medically important conditions and artifacts that may mimic pathology, and who also know techniques to avoid using ultrasound except when it is considered safe and necessary.
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BASIC PRINCIPLES OF ULTRASOUND
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Ultrasound is produced by sound waves that are emitted and then reflected back at varying densities among the uterus, placenta, amniotic fluid, and fetus in order to create images of them. Higher-density structures, such as bone, reflect sound waves at a greater velocity, causing them to appear hyperechoic (bright). Lower-density structures, such as amniotic fluid, reflect back fewer sound waves and at a lower velocity, causing them to appear anechoic or echolucent (dark).
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Choosing an ultrasound transducer with a higher frequency (such as 4–7 mHz) results in better resolution; however, this comes at a cost to depth of penetration. One example of when a higher-frequency transducer may be beneficial would be to use a transvaginal ultrasound probe (typically 5–9 mHz) to assess cervical length because of the proximity of the specific structure being evaluated. In contrast, an ultrasound transducer with a lower frequency (such as 2–5 MHz) is beneficial ...