Guidelines for Performance of the Obstetric Ultrasound
WHEC Practice Bulletin and Clinical Management Guidelines for healthcare providers. Educational grant provided by Women's Health and Education Center (WHEC).
The recent technologic advances in ultrasound imaging, the use of high frequency transvaginal probes, and the potential for chromosomal screening in the first trimester of pregnancy (e.g., nuchal translucency) have only magnified the interest in the use of sonographic imaging in the obstetric patient. Although the almost universal enthusiasm for this modality is exciting, it has raised a new series of questions and problems. It has now been nearly 5 decades since sonography was first used to evaluate the obstetric patient. At first the questions that this modality sought to answer were quite basic. At its inception it was difficult to convince clinicians as to the usefulness of this new diagnostic modality. Now, it is not unusual for a patient to have one or even several ultrasound examinations during her pregnancy. The appeal of ultrasound examination is that it is a non-invasive, safe procedure that has a high degree of patient acceptance and can yield a wealth of information.
The purpose of this document is to present evidence regarding methodology, indications, benefits, and risks of obstetric ultrasonography in specific clinical situations. Sections of the document addressing physician qualifications and responsibilities, documentation, quality control, infection control and patient education are the recommendations of Women's Health and Education Center (WHEC). We are hopeful that this text will serve two purposes: to educate and to intrigue. Through the education process of the healthcare providers will help alleviate errors in diagnosis. Casual use of ultrasonography, especially during pregnancy, should be avoided. Before an ultrasound examination is performed, patients should be counseled about the limitations of ultrasonography for diagnosis.
Introduction and Instrumentation:
It has been suggested that all patients be offered routine ultrasound screening. It has been shown, for example that 90% of infants with congenital anomalies are born to women with no risk factors. However, several studies conducted between 1985 and 1994 found routine ultrasound screening yielded no consistent impact on perinatal mortality or morbidity. In the screened population, the detection rate for congenital anomalies ranged from 16% to 85%. A subsequent secondary analysis of these studies concluded that routine screening was cost-effective. Using a mathematical model to evaluate further the published results, other researchers concluded that routine screening at tertiary centers would be cost-effective, but screening in non-tertiary centers resulted in a net loss (1). In some countries, as many as 90% to 100% of women seeking obstetric care will have at least one ultrasound examination during pregnancy. If ultrasound evaluation is relatively safe and non-invasive and has the potential for yielding important diagnostic information, then why not use this modality in every pregnant patient? As one might imagine, there is much controversy over this single issue.
Most ultrasound machines utilize phased-array real-time technology. The most common transducers, which are the "work horses" of the ultrasound laboratory, are a linear array, a sector transducer (3 to 7 MHz) for abdominal transducers, or vaginal transducers, with frequency ranging from 5 to 10 MHz are used. The higher-frequency transducers are most useful in achieving high-resolution scans, and the lower-frequency transducers are useful in those circumstances in which increased penetration of the sound beam is necessary. Variations of transducer technology include convex linear transducers and multi-frequency probes. Doppler technology and color Doppler flow imaging are areas of present investigations. Two measurements of acoustic output are displayed on-screen with contemporary ultrasound equipment. The thermal index is an estimate of possible tissue temperature increase that may be caused by ultrasound absorption. The mechanical index is a measure of the interaction of ultrasonography with microscopic gas bubbles that are present in all tissues. The mechanical index incorporates cavitations with other possible non-thermal effects of ultrasonography. When the thermal index and mechanical index are adjusted by the user to values of less than unity, the likelihood of tissue effect is very low (2). Most machines allow adjustment of output and will instantly recalculate and display the new thermal index and mechanical index. Most manufacturers now offer machines capable of 3-dimensional surface rendering of fetal anatomy, and some offer near real-time 3-dimensional imaging.
Safety of Ultrasound Examination:
Although there is no reliable evidence of physical harm to human fetuses from diagnostic ultrasound imaging, public health experts, clinicians, and industry representatives agree that casual use of ultrasonography, especially during pregnancy, should be avoided. Viewed in this light, exposing the fetus to ultrasonography with no anticipation of medical benefit is not justified (3). The U. S. Food and Drug Administration views the promotion sale, or lease of ultrasound equipment for making "keepsake" fetal videos are and unapproved use of a medical device; use of ultrasonography without a physician's order may be a violation of state or local law or regulations regarding the use of a prescription medical device. Specific indications are the best basis for ultrasonography in pregnancy (4). The major biologic effects of ultrasonography are believed to be thermal (a rise in temperature) and cavitation (production and collapse of gas-filled bubbles). Although it has been shown that a rise in temperature of less than 1o C may occur during diagnostic ultrasound evaluation, this is unlikely to have any clinical impact in humans. Likewise, cavitation (which requires the preexistence of stable gas-filled nuclei) may occur in humans. In the past 5 years, there has been concern raised over the use of pulsed Doppler imaging when applied to the developing embryo. Spectral or pulsed Doppler imaging uses high-amplitude transmit pulses due to the fact that the signal reflected from blood is small. All of those factors when taken together mean that spectral Doppler has a high likelihood of producing a bioeffect in tissue. Although the potential for embryonic effects from Doppler imaging exists, there is little evidence that ultrasound is teratogenic.
In an attempt to answer these concerns over the safety of ultrasound, studies are initiated to determine if there is an association between diagnostic ultrasound imaging in utero and chromosomal abnormalities, altered fetal growth, learning disabilities, or even malignancy. Two of the most often cited long-term studies evaluating the effect of ultrasound on the fetus are the work of Stark et al and Salvensen et al. Stark et al studied 425 children at birth and between 7 and 12 years of age who were exposed to diagnostic ultrasound in utero. Birth-weight, Apgar scores, and neurologic and cognitive testing revealed no biologically significant differences between exposed and unexposed children. Salvensen et al found no difference in dyslexia between ultrasound-exposed and control groups. At the present time, based on available studies, there is little evidence to indicate ultrasonography as causing abnormalities in the human fetus. The major difficulties with the studies investigating a possible deleterious effect of diagnostic ultrasound evaluation are threefold: 1) experimental ultrasound exposure levels often far exceeded those that are normally used diagnostically; 2) the systems used to show ultrasound effect (plants, cell culture, laboratory animals) may not be applicable to humans; and 3) many studies that have demonstrated adverse effects in vitro have not been reproducible.
Types of Examinations:
The American College of Obstetricians and Gynecologists uses the terms "standard," "limited," and "specialized" to describe various types of ultrasound examinations performed during the second or third trimesters. Although the standard and limited examinations are defined by their components, the specialized examination is defined by the indications for the examination, that is, the circumstances that suggest a more thorough ultrasound examination is needed (5). First-trimester obstetric ultrasonography is distinct from these and is discussed separately.
Standard Examination: It is performed during the second or third trimester of pregnancy. It includes an evaluation of fetal presentation, amniotic fluid volume, cardiac activity, placental position, fetal biometry, and an anatomic survey. If technically feasible, the uterus and adnexa also are examined. Fetal anatomy, may be assessed adequately by ultrasonography after approximately 16-20 weeks of gestation. It may be possible to document normal structures before this time, although some structures can be difficult to visualize because of fetal size, position, movement, abdominal scars, and increased maternal wall thickness. A second- or third-trimester ultrasound examination may pose technical limitations for an anatomic evaluation because of imaging artifacts from acoustic shadowing. When this occurs, the report of the ultrasound examination should document the nature of this technical limitation. Essential elements of fetal anatomic ultrasound survey are (6): Head and Neck -- cerebellum, choroid plexus, cisterna magna, lateral cerebral ventricles, midline falx, and cavum speti pellucidi; Chest -- the basic cardiac examination includes a 4-chamber view of the fetal heart. If technically feasible, an extended basic cardiac examination also can be attempted to evaluate both outflow tracts; Abdomen -- stomach (presence, size, and situs), kidneys, bladder, umbilical cord insertion site into the fetal abdomen, umbilical cord vessel number; Spine -- cervical, thoracic, lumbar, and sacral spine; Extremities -- legs and arms (presence or absence); Sex -- for evaluation of multiple gestations.
Limited Examination: A limited examination is performed when a specific question requires investigation. In an emergency, for example, a limited examination can be performed to evaluate heart activity in a bleeding patient. This evaluation also would be appropriate for verifying fetal presentation in a laboring patient; however, in most cases, a limited examination is appropriate only when the patient has had a prior complete examination.
Specialized Examination: A detailed or targeted anatomic examination is performed when an anomaly is suspected on the basis of history, biochemical abnormalities or clinical evaluation, or suspicious results from either the limited or standard ultrasound examination. Other specialized examinations might include fetal Doppler, biophysical profile, fetal echocardiography, or additional biometric studies. Specialized examinations are performed by an operator with experience and expertise in such ultrasonography who determines the components of the examination on a case-by-case basis.
The primary goal of ultrasound evaluation in the first trimester is to determine whether the pregnancy is intrauterine and whether the embryo is living. With present day equipment, particularly transvaginal transducers, both of these tasks should be readily accomplished at very early stages of gestation. The following are the indications: to confirm the presence of intrauterine pregnancy; to evaluate a suspected ectopic pregnancy; to define the cause of vaginal bleeding; to evaluate pelvic pain; to estimate gestational age; to diagnose or evaluate multiple gestations; to confirm cardiac activity; as an adjunct to chorionic villus sampling, embryo transfer, or localization and removal of an intrauterine device; to evaluate maternal pelvic masses or uterine abnormalities; to evaluate suspected hydatidiform mole. The uterus and adnexa should be evaluated for the presence of a gestational sac. If a gestational sac is seen, its location should be documented. The gestational sac should be evaluated for the presence or absence of a yolk sac or embryo, and the crown-rump length should be recorded, when possible. The crown-rump length is a more accurate indicator of gestational age than is the mean gestational sac diameter. Presence or absence of cardiac activity should be reported. With transvaginal ultrasound examinations, cardiac motion usually is observed when the embryo is 5 mm or greater in length. If an embryo is less than 5 mm in length is seen without cardiac activity, an additional ultrasound examination at a later time may be needed to document cardiac activity.
There is no question that, with a careful examination, the true number of embryos can be accurately determined in the first trimester. Amnionicity and chorionicity should be documented for all multiple pregnancies when possible. When multiple gestations are missed using ultrasound assessment, it is usually from a less than optimal first trimester examination. It is for these reasons that some investigators prefer that if one ultrasound examination is to be done concentrating on fetal number, it should be done in the early to middle second trimester of pregnancy. Evaluation of uterus, adnexal structures, and cul-de-sac should be preformed. The presence, location, and size of leiomyomata and adnexal masses should be recorded. The cul-de-sac should be evaluated by ultrasonography for the presence or absence of fluid.
Second- and Third-Trimester Ultrasonography:
Although evaluating the number of fetuses may be difficult during early pregnancy, it should be extremely easy and accurate in the second- and third-trimesters. For multiple pregnancies, additional information should be documented: chorionicity, amnionicity, comparison of fetal sizes, estimation of amniotic fluid volume (increased, decreased, or normal) on each side of the membrane, and fetal genitalia (when visualized). The components of a standard fetal examination are as follows: fetal cardiac activity, number, and presentation should be reported. Abnormal heart rate or rhythm should be reported. A qualitative or semi-qualitative estimate of amniotic fluid volume should be reported. Although it is acceptable for experienced examiners to estimate the amniotic fluid volume qualitatively, semi-qualitative methods also have been described for this purpose (eg, amniotic fluid index, single deepest pocket, 2-diameter pocket). The placenta's location, appearance, and relationship to the internal cervical os should be recorded. The umbilical cord should be imaged, and the number of vessels in the cord should be evaluated when possible. Transabdominal, transperitoneal or transvaginal views may be helpful in visualizing the internal cervical os and its relationship to the placenta. Gestational age should be assessed. First-trimester crown-rump measurement is the most accurate means of ultrasound dating of pregnancy. Beyond this period, a variety of ultrasonographic parameters, such as biparietal diameter, abdominal circumference, and femoral diaphysis length, can be used to estimate gestational age. Significant discrepancies between the estimated gestational age and fetal measurements from later ultrasound examinations may suggest the possibility of a fetal growth abnormality, intrauterine growth restriction (IUGR), or macrosomia.
A second-trimester specialized ultrasound examination may be targeted to detect fetal aneuploidy. This type of examination has been offered in some centers for the past several years and is aimed at the detection of a range of minor anatomic features associated with an increased risk of fetal aneuploidy. Advanced maternal age or an abnormal first- or second-trimester multiple marker screen indicating and increased risk for Down syndrome are among the indications for a specialized ultrasound examination. The use of the ultrasound markers developed in high-prevalence patient populations in screening for Down syndrome in a low-risk population in the second trimester currently is premature.
Ultrasound Assessment of Cervical Length: Preterm birth is the main cause of perinatal morbidity and mortality. A short cervical length on transvaginal ultrasonography has been shown to be one of the best predictors of spontaneous preterm birth. Screening for a short cervical length has been studied in several populations, including asymptomatic women with singleton gestations at either low or high risk for preterm birth, multiple gestations, and symptomatic women with either preterm labor or preterm premature rupture of the membranes. In a study of 188 women with histories of a prior preterm delivery who underwent serial endovaginal sonography between 22-24 weeks of gestation, Durnwald and colleagues found that, of the women with cervixes shorter than 25 mm, 36% delivered before 35 weeks (7). Another study of 69 women with prior preterm delivery and serial endovaginal sonography every 2 weeks between 16 and 30 weeks of gestation showed that women at less than 20 weeks with cervical length less than the 10th percentile (22 mm) or funneling of internal os had an increased risk of recurrent preterm birth, with 33% delivering within 2 weeks, 66% delivering within 4 weeks, and 100% delivering before 35 weeks. In summary, these studies suggest that, for women with prior spontaneous preterm births, evaluation of the cervical length serially (every 2 weeks) between 16-24 weeks of gestation may aid in the identification of those at increased risk for recurrent preterm birth (8).
Cervical ultrasonography has yet no appreciable clinical usefulness in low-risk populations because of its poor positive predictive value and the absence of preventive therapy. Gestational age at which transvaginal ultrasound cervical length is measured significantly affects the calculation of risk of spontaneous preterm birth. The spontaneous preterm birth risk increases as the length of the cervix declines and as the gestational age decreases. These spontaneous preterm birth risks are important for counseling and management for women with various degrees of short cervical length at different gestational age (9).
Combined Data Comparing Menstrual Age With Mean Gestational Sac Diameter, Crown-Rump length and HCG Levels:
Predicted Biparietal Diameter (BPD) and Weeks' Gestation From the Inner (IOD) and Outer (OOD) Orbital Distances:
Estimate of Fetal Weight (g) Based on Abdominal Circumference and Femur Length:
Chart 1 | Chart 1 Continued
Non-Medical Use of Obstetric Ultrasonography:
The American Institute of Ultrasound in Medicine (AIUM) advocates the responsible use of diagnostic ultrasound. The AIUM strongly discourages the non-medical purposes (eg, solely to create "keepsake" photographs or videos) for psychosocial or entertainment purposes. The use of either two-dimensional (2D) or three-dimensional (3D) ultrasound to only view the fetus, obtain a picture of the fetus or determine the fetal gender without a medical indication is inappropriate and contrary to responsible medical practice. Although there are no confirmed biological effects on patients caused by exposures from present diagnostic ultrasound instruments, the possibility exists that such biological effects may be identified in the future. Thus ultrasound should be used in a prudent manner to provide medical benefit to the patient (10).
Non-medical ultrasonography may falsely reassure women. Even though centers that perform non-medical ultrasonography and create "keepsake" photographs and videos of the fetus may offer disclaimers about the limitations of their product, customers may interpret an aesthetically pleasing image or entertaining video as evidence of fetal health and appropriate development. Ultrasonography performed for psychosocial or entertainment purposes may be limited by the extent and duration of the examination, the training of those acquiring the images, and the quality control in place at the ultrasound facility. Women may incorrectly believe that the limited scan is, in fact, diagnostic. Abnormalities may be detected in settings that are not prepared to discuss and provide follow-up for concerning findings. Without the ready availability of appropriate prenatal health care professionals, customers at sites for non-medical ultrasonography may be left without necessary support, information, and follow-up for concerning findings. Obstetric ultrasonography is most appropriately obtained as part of an integrated system for delivering prenatal care.
Ultrasound examination is an accurate method of determining gestational age, fetal number, viability, and placental location. Gestational age is most accurately determined in the first half of pregnancy. The ability of ultrasonography to diagnose major fetal anomalies is well established. The diagnosis of fetal growth abnormalities with ultrasonography is not precise. Ultrasonography is safe for the fetus when used appropriately. Specific indications are the best basis for the use of ultrasonography in pregnancy. The optimal timing for a single ultrasound examination in the absence of specific indications for a first-trimester examination is at 16-20 weeks of gestation. Serial ultrasonograms to determine the rate of growth should be obtained approximately every 2-4 weeks. Casual use of ultrasonography, especially during pregnancy, should be avoided. Before an ultrasound examination is performed, patients should be counseled about the limitations of ultrasonography for diagnosis. Appropriate documentation of an obstetric or gynecologic ultrasound examination is essential to both direct clinical care and quality assurance. Ideally, quality control is accomplished through careful recordkeeping of obstetric ultrasound examination results, reliable archival of reports and images, and clinical correlation with clinical outcomes. Any practice active in obstetric ultrasonography should maintain such records and make every effort to correlate imaging results with ultimate clinical outcome data.