Key Points
In most cases, ultrasound is the method of choice for imaging the fetus, and the majority of pregnant women in the United States undergo at least one ultrasound.
Sonographer skill and experience play a great role in the accuracy of ultrasound.
Ongoing quality assurance is important.
There have been exciting advances in the field of prenatal imaging within the past few years including three-dimensional ultrasound and fetal magnetic resonance imaging, and the future holds the promise of great breakthroughs.
It is expected that imaging modalities will continue to improve, and it is hoped that techniques utilized in the fields of noninvasive prenatal diagnosis will continue to advance.
Accurate prenatal diagnosis of fetal abnormalities improves patient care by optimizing patient counseling and allowing for informed patient and physician decision-making.
The development and practice of fetology has been dependent on advances in the field of prenatal imaging. Without the ability to accurately visualize the structure and well-being of the fetus within its own intrauterine environment, it would not be possible to diagnose or treat the range of abnormalities that can now be addressed by the multidisciplinary fetal health care team. Rapid advances in the technologic basis of two imaging methods—ultrasonography and magnetic resonance imaging (MRI)—have resulted in highly accurate visualization of the fetal anatomy.
Ultrasonography in obstetrics was first introduced in the late 1950s and has since become the method of choice for imaging the fetus. Gray-scale imaging became available in 1973 and resulted in an enhancement of the ability to differentiate the appearance of various organs and tissue interfaces. The advent of real-time sonographic scanning during the 1970s was vital for the accurate visualization of the constantly moving fetus. The subsequent development of higher frequency transabdominal and transvaginal transducers in the late 1970s resulted in vast improvements in the resolution of fetal images and also pushed back the gestational age barrier for accurate prenatal diagnosis into the late first and early second trimesters (D’Alton, 1998).
The 1980s saw innovations in obstetric sonographic technology, including the use of pulsed and color Doppler sonography, which allowed for detailed analysis of fetal perfusion and improvements in the visualization of fetal cardiac anatomy. More recent advances include the development of power Doppler, which displays the strength of a Doppler signal rather than the direction of flow. This technique is useful in fetal imaging for low-flow states and may aid in the definition of fetal tumors and assessing placental function (Ogle and Rodeck, 1998). Three-dimensional ultrasonography is now also available for fetal imaging and has revolutionized the field of prenatal imaging by allowing simple sonographic acquisition of a “block” of fetal tissue (Timor-Tritsch and Platt, 2002). Postacquisition computer processing allows reconstruction of any number of planes through the organ of interest as well as producing real-time surface rendering images of the fetus.
Ultrasonographic imaging is an integral part of obstetric practice today. In the United States, it is performed in the majority of all pregnancies (Horger and Tsai, 1989; Goncalves and Romero, 1993; Martin et al., 2002). Sonography has been used routinely for accurate dating of pregnancy, confirmation of pregnancy location and number of gestations, prenatal diagnosis of congenital malformations, and assessment of fetal well-being (American College of Obstetricians and Gynecologists, 2008). With the increasing availability of ultrasound equipment, the performance of obstetric ultrasonography has grown to the point that, in some countries, one or more ultrasound examinations are recommended during all pregnancies (Royal College of Obstetricians and Gynaecologists, 1994).
Studies evaluating the performance of ultrasonography in pregnancy have yielded conflicting results. A comprehensive meta-analysis of more than 30 studies assessing the performance of routine prenatal ultrasound in screening for fetal malformations reported detection rates ranging from 13% to 82% with a mean of 27.5% (Levi, 2002). Detection rates have been shown to vary according to the nature and severity of the anomaly with higher detection rates achieved for major structural malformations than for minor malformations (Grandjean et al., 1999). The inconsistent performance of the second trimester anatomic survey may also be explained by variations in the populations screened and differences in the facilities at which the ultrasounds are performed. For example, both the Helsinki and RADIUS trials demonstrated significantly higher detection rates when screening ultrasounds were performed at tertiary care facilities than when the ultrasounds were performed at community-based facilities (Crane et al., 1994; Saari-Kemppainen et al., 1994).
Apart from the detection of congenital abnormalities, obstetric ultrasonography can be expected to be beneficial in other aspects of pregnancy management. The routine use of obstetric ultrasonography has been shown to reduce the rate of postterm pregnancies and to reduce the use of tocolytic medications, both because of an improvement in the accuracy of pregnancy dating (Saari-Kemppainen et al., 1990; LeFevre et al., 1993). In addition, the routine use of obstetric ultrasonography has been shown to significantly increase the early detection of multiple gestations, which is essential if appropriate modifications in pregnancy management are to be applied (LeFevre et al., 1993).
While the RADIUS Trial did not demonstrate any improvement in overall perinatal outcome from routine obstetric ultrasonography, this study has been criticized because of its poor performance in the detection of anomalies (Ewigman et al., 1993; Goncalves and Romero, 1993). Other studies have demonstrated a significant reduction in perinatal mortality following routine obstetric ultrasonography, mostly because of an increase in the rate of pregnancy termination for congenital anomalies (Saari-Kemppainen et al., 1990). While an initial meta-analysis of four randomized clinical trials of routine versus indicated ultrasonography also confirmed a significantly lower perinatal mortality rate in patients allocated to routine scanning (Bucher and Schmidt, 1993), another more recent review failed to confirm a perinatal mortality advantage in the general population from routine obstetric ultrasonography (Neilson, 1998).
Perhaps the most likely reason for the significant discrepancy in the sensitivities of ultrasonography for the detection of anomalies is the effect of the skill and experience of the sonographer. There is undoubtedly wide variation in the skill levels of sonographers between different centers that practice obstetric ultrasonography. In the RADIUS Trial, for example, there was a significant difference in the rates of anomaly detection between participating tertiary and nontertiary level centers (Crane et al., 1994). Nontertiary level centers detected only 13% of congenital anomalies in the RADIUS Trial and were unable to detect any craniofacial, cardiac, gastrointestinal, or skeletal malformations. Tertiary level centers performed significantly better, detecting 35% of anomalies (Goncalves and Romero, 1993). Other studies have also demonstrated that in expert hands, second trimester ultrasound can detect more than 70% of fetal malformations (Boyd et al., 1998; Van Dorsten et al., 1998).
In a retrospective study from Vienna, the impact of sonographer experience was demonstrated by a wide variation in rates of anomaly detection between different centers with different skill levels (Bernaschek et al., 1996). In that study, the overall anomaly detection rate was only 22% for obstetric ultrasonography performed in private obstetricians’ offices, 40% in general hospitals, and 90% for ultrasonography performed by experts in fetal imaging.
Because of the clear association between the skill or experience of the sonographer and rates of anomaly detection, it has been suggested that obstetric ultrasonography should be performed only in tertiary level centers, where up to four times as many fetuses with anomalies may be detected (DeVore, 1994). However, it is not clear that all pregnant patients will have equal access to tertiary level centers, which may be especially problematic in rural or other underserved areas.
Given the influence of the sonographer’s skill and experience on the rate of anomaly detection for obstetric ultrasonography, it is logical to expect that basic standards and requirements for the performance of ultrasonography be delineated. However, in the United States there is no mandatory requirement for licensure or accreditation for practitioners who provide obstetric ultrasonography services. Minimum standards are described only in the form of suggested guidelines, and any form of accreditation is entirely voluntary in nature.
The American Institute of Ultrasound in Medicine has suggested minimum qualifications for all practitioners involved in obstetric ultrasonography (American Institute of Ultrasound in Medicine, 1993). One of the key components of such qualification is the completion of an approved residency program, or the performance and interpretation of at least 500 obstetric ultrasound examinations. Furthermore, the American Institute of Ultrasound in Medicine now accredits obstetric ultrasonography practices, which involves evaluation of the credentials of sonographers, quality of fetal images, type of ultrasound equipment, methods of data storage, and presence of quality control measures in place (Abuhamad et al., 2004; American Institute of Ultrasound in Medicine, 2005).
Theoretical safety risks from ultrasound energy include thermal damage or cavitation, with subsequent tissue injury. Most prenatal ultrasound examinations produce energies of 10 to 20 mW/cm2, which is well below the arbitrarily defined safe cutoff level of 100 mW/cm2(American College of Obstetricians and Gynecologists, 2008). Newer prenatal imaging methods, such as those that use power Doppler, may be associated with higher energy outputs, which reinforce the need to continuously monitor power output during prenatal ultrasonography and to achieve the lowest possible energy exposure.
Randomized trials on the safety of diagnostic ultrasonography in pregnancy have demonstrated no significant differences in developmental, neurologic, or psychologic outcomes, with up to 12 years of follow-up (Stark et al., 1984). One study did demonstrate a nonsignificant increase in the frequency of dyslexia in the group exposed to ultrasound in utero, as compared with the group with no ultrasound exposure (Stark et al., 1984). A subsequent study evaluating school performance and dyslexia in groups exposed and not exposed to ultrasound in utero found no differences in any outcome measures, although there was an association between left-handedness and in utero ultrasound exposure (Salvesen et al., 1992, 1993). One study has also suggested an association between frequent ultrasound exposure in utero (five ultrasound examinations) and growth restriction, although this has not been confirmed by other investigators (Newnham et al., 1993).
Overall, it appears that routine ultrasonography in pregnancy is not associated with any adverse outcome for the fetus. Repeat ultrasound examinations should be performed only as indicated.
The four main roles of prenatal ultrasonography in contemporary obstetric practice are
to confirm fetal gestational age and number;
to search for fetal malformation;
to confirm fetal well-being; and
to aid in the performance of invasive diagnostic and therapeutic fetal procedures.
The ability of prenatal ultrasonography to accurately confirm fetal gestational age and number of fetuses is self-evident. The RADIUS Trial demonstrated a significantly decreased chance of post-term induction of labor following accurate sonographic dating of pregnancy and a significant increase in the prenatal detection of multiple gestations (LeFevre et al., 1993).
The ability of prenatal ultrasonography to diagnose fetal malformations, both as a screening tool and for targeted examinations, has also been confirmed. In expert hands, screening ultrasonography can be expected to detect approximately 70% of all fetal malformations (Boyd et al., 1998; Van Dorsten et al., 1998). However, the detection rate for individual anomalies varies significantly. While almost all spinal, renal, and abdominal wall malformations are detected by screening prenatal ultrasonography, the detection rate for isolated cardiac defects is only approximately 50% (Boyd et al., 1998; Flood and Malone, 2008). Efforts to increase the second trimester detection rate of Down syndrome by including soft markers for aneuploidy (such as short femur, short humerus, echogenic bowel, nuchal thickening, and choroid plexus cysts) may be successful, although it is likely that any such improvement will be accompanied by an increase in the false-positive rate (Nyberg et al., 2001; Bromley et al., 2002; Filly et al., 2004). See Chapters 2 and 3 for further discussion of first and second trimester screening for Down syndrome.
Prenatal ultrasonography also plays a crucial role in the confirmation of fetal well-being, especially following the identification of a fetal abnormality or a condition associated with a high risk of adverse fetal outcome. Fetal biophysical profile and pulsed Doppler assessment of fetal arterial flow may be used for identifying fetuses with compromised reserve that may benefit from either intensive surveillance or elective premature delivery (American College of Obstetricians and Gynecologists, 2008).
The ability to perform invasive prenatal diagnostic procedures safely, as well as fetal therapy, has been significantly improved by real-time ultrasonography (American College of Obstetricians and Gynecologists, 2008). Diagnostic procedures during the first trimester, such as chorionic villus sampling and embryofetoscopy, are now commonly performed under ultrasound guidance. Amniocentesis, fetal blood sampling, vesicocentesis, thoracentesis, and fetal biopsies are also possible because of the availability of real-time ultrasonography. The ability to treat the fetus using blood transfusions, drug infusions, and shunt placement is now also possible as a result of advances in sonographic technology and availability.
Three-dimensional (3D) ultrasound allows for multiplanar imaging enabling the examiner to move back and forth between different planes due to the capability of viewing the fetus in three rather than two spatial planes (Figure 1-1). Images can be reconstructed and the examiner can move the fetus into ideal desired positions that are often not possible with conventional ultrasound. In addition, 3D scanning enhances imaging capabilities by permitting surface rendering of a structure. Acquisition of data points through the entire volume of interest is required to produce 3D ultrasound pictures. Acquisition quality depends on acquisition speed. Slow speeds result in more scanned slices and are used for nonmoving organs. Fast speeds are preferable for moving structures. The “four-dimensional” (4D) real-time imaging technique requires ultrafast acquisition. 4D ultrasound displays a continuously updated and newly acquired volume in any rendering modality. This creates the impression of a moving structure (Timor-Tritsch and Platt, 2002).
Previously, it was thought that 3D ultrasound provides only aesthetic images without contributing to prenatal diagnosis. There are no accepted indications for 3D ultrasound in obstetric practice at this point. Few outcome studies have confirmed whether or not this technology changes practices or clinical outcomes (Lee, 2003; American College of Obstetricians and Gynecologists, 2008). Nonetheless, this technology is rapidly advancing and has been shown to be helpful. It appears to be useful as an adjunct to 2D ultrasound in fetal echocardiography and in the diagnosis and further evaluation of certain fetal anomalies such as cleft lip and palate and skeletal anomalies (Dyson et al., 2000; Garjian et al., 2000; Johnson et al., 2000; Sepulveda et al., 2003; Roman et al., 2004; Sklansky et al., 2004; Merz and Welter, 2005). Goncalves et al. (2006) recently conducted a study that evaluated whether or not conventional ultrasound adds important information to 3D/4D imaging (Goncalves et al., 2006). Fifty-four fetuses without abnormalities and 45 fetuses with 82 abnormalities diagnosed by 2D ultrasound were evaluated. Agreement between 3D/4D and 2D ultrasound occurred in 90.4% of cases. Six anomalies were missed by 3D/4D when compared to 2D ultrasound. There were also two discordant diagnoses. There was one abnormality suspected by 3D/4D ultrasound, which was not confirmed by 2D ultrasound. The sensitivity and specificity of 3D/4D ultrasound and 2D ultrasound was 92.2% and 76.4% and 96.1% and 72.7%, respectively. This was not found to be a statistically significant difference. The authors concluded that the findings of 3D/4D ultrasound are consistent with the findings from 2D ultrasound.
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