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Only ultrasonographers or other health care professionals trained in ultrasonography should perform ultrasound examinations.undefined#ref8">8
Ultrasound during pregnancy is a common and safe diagnostic procedure that uses sound waves to create an image of the fetus and placenta.2 The image produced–the sonogram–provides visual assistance that improves safety during invasive tests.3,5,8
The sonogram also provides clinical information about (Table 1):
When the sound waves of an ultrasonic beam are aimed at body tissues, they are deflected by structures of different densities in their path and returned as echoes. In obstetrics, the ultrasonic beam sent by a transducer is directed through tissues of the abdomen or vagina to provide a two- or three-dimensional image. The majority of obstetric ultrasounds are performed using two-dimensional images (Figure 1).8
During pregnancy, a transabdominal, transvaginal, or transperineal ultrasound examination may be performed.3,8
Transabdominal and transvaginal ultrasound may be used in conjunction during the second and third trimesters to evaluate painless vaginal bleeding after 20 weeks’ gestation and rule out placenta previa.6 A transabdominal ultrasound examination should be performed initially and be followed by a transvaginal ultrasound unless the transabdominal scan clearly shows that the placenta is not located in the lower uterine segment.6 Because of differences in imaging techniques, the transabdominal ultrasound should be performed first with the patient having a full bladder, and the transvaginal ultrasound should be performed after the patient has emptied her bladder.8
Ultrasonography may provide early diagnoses, allowing more options early in pregnancy. An early diagnosis of a fetal anomaly, for example, allows the family to have choices such as intrauterine surgery, other treatment therapies, termination of the pregnancy, or time to prepare for the care of a newborn with a disorder.3,8 However, when an ultrasound examination is performed, patients should be counseled about its limitations. This should include a discussion of the sensitivity of the examination for the detection of abnormalities and potential false-positive findings.3
Obstetric ultrasound examination has three levels:
The use of ultrasound for gestational dating is common in most pregnancies. Fetal age estimated by crown-rump length in the first trimester is the most accurate means for ultrasound dating of pregnancy. Starting at 14 weeks’ gestation, the biparietal diameter, abdomnal circumference, and femur length can be used to help estimate gestational age.3 The optimal time for a single ultrasound exam is 18 to 22 weeks’ gestation3 because this period can allow a survey of fetal anatomy and an accurate estimation of gestational age. If an accurate earlier scan is obtained, the gestational age should not be revised using a scan done later in pregnancy. After 22 weeks’ gestation, ultrasound dating is less reliable because of variability in the size of the fetus.3,8
Ultrasound procedures use real-time scanning that shows movement as it happens, allowing the observer to see fetal heart motion, fetal breathing activity, and fetal body movement. Real-time scanning also allows the observer to distinguish between moving tissues of the fetus and relatively fixed maternal tissues. Advances in technology and software now can produce a three-dimensional image with greater clarity and visual depth than the two-dimensional image (Figure 2).8 Three-dimensional ultrasonography has not been found to provide a clinical advantage over two-dimensional ultrasound, but it may be helpful in identifying fetal facial anomalies, neural tube defects, fetal tumors, and skeletal malformations.3
Rationale: The patient is tilted to one side to avoid supine hypotension, which can result from the gravid uterus compressing the vena cava and aorta when the patient is supine.
Rationale: Adequate exposure allows the ultrasonographer to obtain the angles needed to evaluate the uterus and surrounding structures.
Rationale: Abnormalities should be brought to the obstetric practitioner’s attention so appropriate interventions can be performed, and further testing can be scheduled, if indicated.
Rationale: An empty bladder helps the ultrasonographer obtain a better view of pelvic structures during a transvaginal ultrasound.
Rationale: The patient is tilted to one side to avoid supine hypotension, which can result when the gravid uterus compresses the vena cava and aorta when the patient is supine.
7 A wedge is not necessary during the first trimester because the small size of the fetus poses a low risk of supine hypotension.
Rationale: The nonpowdered glove is used as a probe cover for hygiene purposes. Powdered gloves should not be used as probe covers because of the potential for reverberations of image quality.
American College of Obstetricians and Gynecologists (ACOG). (2014, reaffirmed 2019). Practice bulletin no. 145: Antepartum fetal surveillance. Obstetrics & Gynecology, 124(1), 182-192. doi:10.1097/01.AOG.0000451759.90082.7b
Roberts, V. and others. (2016). Utilization and safety of contrast-enhanced ultrasound in pregnancy. AJOG: American Journal of Obstetrics & Gynecology, 214(1, Suppl.), S39. doi:10.1016/j.ajog.2015.10.073
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