Pediatr Cardiol 25:223–233, 2004 DOI: 10.1007/s00246-003-0588-y

Technique of Fetal Echocardiography L. Allan King’s College Hospital, London, United Kingdom

Abstract. The fetal heart is examined by a series of sequential ultrasound views, which image the cardiac connections and intracardiac structure. The imformation obtained by cross-sectional imaging can be augmented by the use of cardiac measurements, and evaluation with pulsed or color flow Doppler. A thorough familiarity with the normal appearance and systematic evaluation of standard views can exclude or diagnose major heart malformations. With training, cardiac evaluation can be performed during routine obstetric scanning, in a matter of minutes. An accurate description of any abnormal findings and a knowledge of the types of cardiac malformation which are possible, will help the examiner to reach a correct diagnosis. Keywords: Fetal heart — Fetal echocardiography — Congenital heart disease — Prenatal diagnosis Ultrasound equipment for real-time imaging of the fetus became available in obstetrics during the late 1970s. At the same time, ultrasound evalution of the heart was introduced into pediatric cardiology and became an important diagnostic tool. The conjunction of these two advances led to the description of the appearance of the fetal heart, as seen by crosssectional imaging, by several workers by 1980 [4, 7– 9]. As image quality has improved with advancing technology, and as techniques and experience have improved, the details of the fetal heart anatomy that can be identified have expanded from these early days. The aim of fetal heart scanning is to exclude congenital heart disease or to make accurate prenatal diagnosis if a malformation is present. Currently, for fetal heart examination a transvaginal transducer can be used as early as 9 weeks of gestation [3]. The connections of the fetal heart can begin to be detected abdominally by 11 weeks of gestation [6] and are reliably detected in most patients by 14 weeks. However, the usual timing for complete fetal heart Correspondence to: L. Allan

scanning is approximately 18–20 weeks. At 18 weeks of gestation, all the cardiac connections can be seen in almost every patient, although in some cases image quality limits the accuracy in excluding minor lesions. Sometimes, in obese patients, a scan after 20 weeks may be necessary to identify all the connections. Although the quality of cardiac images tends to be best between approximately 24 and 28 weeks of gestation, this is too late for the first scan to be performed.

Technique Success in fetal heart scanning involves skill in obtaining the optimum window, which is in turn dependent on a willingness to explore the maternal abdomen in order to approach the heart from different orientations to find the ideal, coupled with a thorough familiarity of cardiac views whatever the fetal position. In addition, standard views must not only be obtained correctly but also be properly and completely analyzed. It should first be established that the stomach is on the left side of the fetus and on the same side as the apex. This can be readily checked by aligning the transducer in the long axis of the fetus, with the head to the right of the screen [5]. The transducer should then be turned through 90° clockwise. If the fetus lies with the back down, the left of the fetus will be on the right of the screen; if the fetus lies with the back up, the left side is on the left of the screen. The transverse views of the heart are obtained by sweeping the ultrasound beam from the abdomen to the inlet of the thorax, as illustrated in Fig. 1. These views will usually demonstrate all the features that are necessary for defining a normal heart, whether during a routine screening examination of the fetal anatomy or as part of detailed fetal echocardiography. The four-chamber view should be analyzed in detail before continuing a cranial sweep in the transverse orientation to image the great artery connections, the duct, and the aortic arch. These transverse views are sufficient to identify

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3. Aortic origin (five-chamber view) 4. Pulmonary outflow tract (three-vessel view) 5. Transverse aortic arch Long-axis views 1. 2. 3. 4. 5.

Short axis of the left ventricle (LV) Tricuspid/aortic cut Long axis of the duct Long axis of the aortic arch Inferior (IVC) and superior vena cava (SVC)

Angulated views 1. Long axis of the LV 2. Arch and duct simultaneously Fig. 1. The fetus is seen in a long-axis projection. The transducer is seen on the maternal abdomen. The beam is swept in a horizontal plane from the stomach up to image the four-chamber view, the aortic outflow, the pulmonary outflow tract, and transverse arch sequentially as illustrated. It can be seen that a small change in the angle of the transducer can produce all these cardiac views.

a normal heart, and additional views are not essential. However, if the fetal position prevents ideal transverse views, or if an anomaly is found, views obtained in the long axis of the fetus may contribute to the evaluation. Thus, it is important to be familiar with obtaining and analyzing all the cardiac views. The long-axis views of the fetus similarly involve sweeping the ultrasound beam across the anterior chest to image sections that illustrate the ventricular chambers and great arteries. In addition, there are two views for which the transducer is angulated and that lie between the transverse and longitudinal views. In the early fetus, sweeping from the abdomen to the arch for the transverse views, or across the front of the chest for the long-axis views, only requires a small change in beam angulation. In the larger, later fetus, the transducer needs to be moved up and down or across the front of the chest (i.e., over the maternal abdomen) to obtain these views. Sequential Analysis The heart should be analyzed in terms of its connections and in terms of the characteristics of each component chamber or vessel. The heart has six connections, three on each side; the venous–atrial, atrioventricular and ventriculoarterial connections. These can all be checked by correct analysis of the transverse sweep. The standard projections that may be used in fetal heart scanning include Transverse views 1. Abdomen at the level of the stomach 2. Four-chamber view

Some of the transverse planes are not strictly horizontal cuts of the fetus but require slight transducer angulation. Similarly, some of the long-axis views are not strictly vertical. They are not all necessary for a complete fetal echocardiogram. Whatever views are obtainable should be used to identify the cardiac connections and to visualize the atrial and ventricular septums, the arch, and the duct. Once these structures have been checked, a study is complete. Transverse Views Transverse View of the Abdomen. This view shows the stomach and the relationship of the aorta and IVC, which denotes atrial situs (Fig. 2). The points to note in this section are 1. Stomach lies on the left, in the middle of the left half of the abdomen. 2. The aorta lies anterior and to the left of the spine. 3. The IVC lies anterior to the aorta and slightly to the right of the midline. 4. The IVC and aorta are similar in size, with the aorta more pulsatile. By sweeping cranially from this section, the stomach can be confirmed to lie on the same side as the cardiac apex, and the IVC can be traced into the right atrium. The IVC joins the right atrium immediately anterior to the posterolateral portion of the atrial septum, close to the mouth of the foramen ovale. Four-Chamber View. This is the most important view in fetal heart scanning because it is abnormal in up to 60% of major heart malformations if it is analyzed correctly [2]. In postnatal life, the apex points caudally, almost toward the left side of the pelvis. In contrast, in the fetus the heart lies in a more horizontal position due to the large fetal liver pushing the apex cranially [4]. Because of this displacement, the four-

L. Allan: Technique of Fetal Echocardiography

Fig. 2. The abdomen is imaged at the level of the stomach. It lies on the left of the midline. The aorta (Ao) lies anterior and slightly to the left of the spine. The inferior vena cava (IVC) lies anterior and to the right of the aorta. These normal relationships denote normal atrial situs.

chamber view is imaged in a horizontal cross section of the thorax, cutting just above the diaphragm. Therefore, it is the easiest view to obtain technically, although the slice must be obtained at precisely the correct level, neither too low, which displays the coronary sinus, nor too high, which displays the aortic origin. In addition, the cut should be truly transverse such that the thorax appears circular and one complete rib is seen. The four-chamber view can be seen in apical or lateral projections. In the apical fourchamber view, the ultrasound beam is almost parallel to the ventricular septum, whether the fetus lies spine up or spine down (Fig. 3). In the lateral four-chamber view, the ultrasound beam is almost perpendicular to the ventricular septum and the spine lies either predominantly to the left or to the right of the screen (Fig. 4). The four-chamber view illustrates the pulmonary venous connection, the atrioventricular connection on both sides of the heart, the atrial septum, and the inlet and muscular portions of the ventricular septum. The four-chamber view must be analyzed in a systematic fashion. The points to note in this view involve evaluation of size, position, structure, and function: Size Normally, the heart occupies approximately one third of the thorax, or is a mean of 0.55 ± 0.05, if the circumference of the thorax is divided by the circumference of the heart (the C/T ratio). Position Normally, the apex points out of the left anterior thorax with the ventricular septum lying at an an-

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Fig. 3. The four-chamber view is obtained in a transverse section of the thorax just above the diaphragm in an apical projection. LV, left ventricle; RV, right ventricle.

Fig. 4. The four-chamber cut is seen looking from the right side of the fetus. This is a lateral four-chamber view. The tricuspid valve is seen lying anterior to the mitral valve. LV, left ventricle; RV, right ventricle.

gle of approximately 45° to the midline of the thorax. Structure 1. There are two atria of approximately equal size. 2. The foramen ovale defect occupies approximately one third of the atrial septum with the flap valve lying in the cavity of the left atrium. 3. There are two atrioventricular valves opening equally. 4. There are two ventricles, which are equal in cavity size and wall thickness. 5. There is an intact crux where the atrial septum, atrioventricular valves, and ventricular septum meet.

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2. 3. 4. 5.

Fig. 5. Moving cranially from the four-chamber view, the aortic origin from the left ventricle (LV) can be seen. The close relationship of the aorta (Ao) to the two atrioventricular valves can be appreciated when performing this move in real time.

6. The septal insertion of the tricuspid valve (TV) is slightly lower than the mitral valve (MV) [Normal ‘‘off-setting’’ of the atrioventricular (AV) valves] 7. There is an intact ventricular septum. 8. Pulmonary veins connect to the LA. 9. The right ventricle (RV) apex contains the moderator band. Function 1. The atria and ventricles contract synchronously. 2. The ventricles contract equally. 3. There is equal flow through the atrioventricular valves on color-flow mapping with no regurgitation. The two ventricular chambers should appear equal in size in the normal fetus. However, in the last 10 weeks of gestation, the right heart can appear larger than the left in a normal fetus. If disproportion is found between the ventricular chambers in late pregnancy, possible causes, such as coarctation of the aorta, total anomalous pulmonary venous return, or intrauterine growth retardation, should be carefully excluded before assuming the finding is within normal limits. Transverse View of the Aortic Origin (Five-Chamber View). Just cranial to the four-chamber view, the aortic origin from the left ventricle can be imaged (Fig. 5). This view illustrates the left-sided ventriculoarterial connection and the perimembranous and muscular parts of the ventricular septum. The points to note in this section are 1. The aorta arises in the center of the chest, in the center of the heart.

The aorta arises between the two AV valves. There is aortic–mitral continuity posteriorly. There is aortic–septal continuity anteriorly. The muscular and perimembranous septum appears intact.

Transverse View of the Pulmonary Trunk and Duct (Three-Vessel View). Immediately cranial to the origin of the aorta (Ao), the pulmonary artery (PA) can be seen arising from the right ventricle (Fig. 6). Moving sequentially between the aortic origin and this section illustrates that the pulmonary valve lies anterior and cranial to the aortic valve, and that the PA crosses over the aortic origin. This section has been designated the three-vessel view by Yoo et al. [12]. Their recommendations of the analysis of this view are a useful addition to detailed heart scanning. These include the following: 1. The three vessels are seen lying from left to right: the PA, the Ao, and the SVC, respectively. 2. They are in descending order of size: PA > Ao > SVC. 3. Each lies slightly posterior to the other: the PA is anterior to the Ao, the Ao is anterior to the SVC. 4. The PA arises close to the anterior chest wall. 5. The pulmonary valve (PV) lies anterior and cranial to the aortic valve. 6. The PA crosses over the aortic origin. 7. The pulmonary trunk and its ductal continuation is directed straight posteriorly. 8. The arterial duct connects to the descending Ao just to the left and in front of the spine. 9. The PA branches into the right pulmonary artery (RPA) and duct. The division of the PA into the duct and RPA is seen in the true horizontal projection of this view. Slight caudal angulation of the ultrasound beam on the left side is necessary to image the left pulmonary artery (LPA) because this vessel takes a downward path from the pulmonary trunk. Transverse View of the Aortic Arch. Moving cranially from the transverse view of the duct, the aortic arch is seen in transverse projection as the most superior vessel in the thorax (Fig. 7). The points to note in this view are 1. The aortic arch lies superior to the transverse view of the duct. 2. The curve of the arch starts in the middle of the chest. 3. The curve of the arch crosses the midline in front of the spine.

L. Allan: Technique of Fetal Echocardiography

Fig. 6. Moving cranial to the aortic valve, the pulmonary valve can be seen lying anterior, leftward, and cranial to the aortic valve. The ductal connection to the descending aorta, aorta in cross section, and superior vena cava (SVC) are seen in this view, which has been termed the three-vessel view. When making the move between the aorta (Ao) and the pulmonary artery (PA) in real time, the pulmonary can be seen to ‘‘cross over’’ the aortic origin. This is because the aorta initially sweeps out toward the right before turning leftward, in contrast to the pulmonary trunk and duct, which courses directly posteriorly to join the descending aorta just to the left of the spine.

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Fig. 7. Cranial to the transverse view of the duct, the aortic arch can be seen in a transverse section. The superior vena cava (SVC) is cut in this section in its short axis, lying tothe right of the aorta.

Transverse View of the Arch and Duct. A slight caudal tilt of the ultrasound beam to the left from the transverse view of the aortic arch allows the aortic arch and duct to be imaged simultaneously (Fig. 8). The two vessels can be compared directly in size and the junction between the descending aorta and the duct, just in front and to the left of the spine, can be noted. The points to note in this view are 1. Duct and arch are of similar size. 2. Duct and arch join distally just in front and to the left of the spine. 3. The direction of blood flow in both vessels is the same. Long-Axis Views Long-Axis View of the Left Ventricle. This view is obtained with the transducer beam at an angle between the transverse and true long-axis views, cutting the fetus between the left side of the abdomen and the right shoulder. It displays the aortic origin from the left ventricle and the perimembranous and muscular parts of the ventricular septum (Fig. 9). The points to note in this view are 1. The Ao sweeps anteriorly, superiorly, and rightwards from the LV. 2. The Ao < PA in size (compared in a subsequent view).

Fig. 8. The transducer is angled from the transverse view of the arch or duct to cut through both the arch and duct simultaneously.

3. There is aortic–mitral continuity posteriorly. 4. There is aortic–septal continuity anteriorly. 5. The muscular and perimembranous interventricular septums appear intact. Short-Axis View of the Left Ventricle. This view is obtained almost parallel and slightly to the left of the sternum. The right ventricular outflow tract and PV are seen anteriorly, with the circular LV posteriorly (Fig. 10). This images the right-sided ventriculoarterial connection, the muscular ventricular septum, and the LV in short axis. Slight rightward angulation of the transducer from this view images the orifice of the MV and slight leftward angulation images the papillary muscles of the MV. The points to note in this view are

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Fig. 9. The transducer is angled from a four-chamber view to cut approximately between the right shoulder and left hip. The aortic origin from the left ventricle (LV) is seen in this section. Ao, aorta; RV, right ventricle.

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Fig. 11. This section cuts between the two fetal shoulders. It lies parallel and just anterior to the ventricular septum and images the aorta as it sweeps slightly anterior and rightward. The right ventricular outflow tract is also seen with the inferior (IVC) and superior vena cavae (SVC) posteriorly. Ao, aorta; RA, right atrium; RV, right ventricle.

abdomen with one shoulder upward. The view is achieved by cutting between the fetal shoulders, just on the right side of the ventricular septum. This images the right atrium, TV, right ventricular outflow tract, and PV with the ascending aorta curving anteriorly into the scan plane from the left side of the heart (Fig. 11). The points to note in this view are

Fig. 10. The longitudinal views involve a sweep across the chest of the fetus. This section is almost parallel and to the left of the sternum and cuts through the left ventricle (LV) in short axis. The right ventricular outflow tract is imaged anteriorly. RV, right ventricle.

1. 2. 3. 4.

1. The pulmonary valve lies anterior and cranial to the aortic origin. 2. The PA > Ao in size (slightly). 3. The ascending aorta forms a curve toward the head. 4. The TV is patent. 5. The inlet and outlet portions of the RV are normal. 6. The SVC and IVC connect to the right atrium. 7. The SVC and IVC are of similar size.

The RV lies anteriorly and is ‘‘tube’’ shaped. The infundibulum supports the pulmonary valve. The LV lies posteriorly and is circular in shape. The MV in the LV has two cusps (‘‘fish-mouth’’ appearance). 5. The MV in the LV is supported by two papillary muscles. 6. The muscular interventricular septum appears intact.

Long-Axis View of the Duct (Short Axis of the Aorta). When the arterial duct is imaged in long axis, the aorta is seen in cross section as a circular structure. This view is obtained by cutting the fetus across the sternum from right to left anteroposteriorly. This view illustrates all the right heart connections, from the IVC to the duct, the left PA, and the atrial septum (Fig. 12). The points to note in this view are

Tricuspid/Aortic View. This view is commonly readily obtained in the fetus as he or she often lies with the spine on the left or right of the maternal

1. The IVC connects to the right atrium. 2. The TV and PV are patent. 3. The Ao is seen as a circle in the center of the section.

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Fig. 12. This section cuts diagonally across the sternum, to the left of the spine inferiorly and to the right of the spine superiorly. It can be seen to cut through the inferior vena cava, the tricuspid valve, the aortic valve, the pulmonary valve, and duct. This is a long-axis view of the arterial duct. Ao, aorta; PA, pulmonary artery; RV, right ventricle.

Fig. 13. This section is angulated to pass from the right side of the sternum toward the left side of the spine. This cuts through the ascending aorta (AAo), transverse arch, and descending aorta (DAo) simultaneously, imaging the aortic arch in long axis. It can be seen that the aortic arch branches are imaged in this section. LA, left atrium; RPA, right pulmonary artery.

4. The PV lies anterior and cranial to the aortic valve. 5. The LA lies between the ascending and descending aorta. 6. The PA connects to the Ao via the duct, forming a wide ductal ‘‘arch.’’ 7. The PA branches inferiorly into the duct and LPA.

Use of Measurements

Long-Axis View of the Aortic Arch. This view is obtained by cutting the fetus in the long axis with transducer angulation from the right of the sternum to the left of the spine. The aortic arch and its three branches—the RPA, the two atria, and the atrial septum—are seen (Fig. 13). The inferior vena caval connection to the right atrium is often seen in this view. The points to note in this view are 1. The aorta arises in the center of the chest. 2. The arch forms a tight ‘‘hook’’ shape. 3. Three head and neck vessels arise from the superior aspect of the arch. 4. The RPA lies below the arch. 5. The RPA is approximately one third of the size of the aorta. 6. The LA lies between the ascending and descending aorta. 7. The foramen ovale flap lies in the LA. 8. Left-sided pulmonary veins enter LA (seen on color-flow mapping). 9. The IVC can often be seen entering the right atrium in this view.

It is not essential to measure any cardiac structure during a normal fetal echocardiogram, although I usually measure the aortic and pulmonary artery size routinely (Figs. 14 and 15). However, if there is disproportion of chamber sizes, or if a structure appears abnormal in size, appropriate measurements should be made for comparison with normal ranges [10, 11] (Figs. 16–19). Measurements in two-dimensional images should be made of the internal dimensions (inner to inner walls) in a standard fashion. The ventricles are measured at their maximum width or in length from the atrioventricular valve ring to the apex. The atrioventricular and arterial valve orifices are measured in diastole. The ventricular walls and septal thickness are measured about midcavity. The C/T ratio is the circumference of the heart divided by the circumference of the thorax. The left and right heart volumes can be computed fairly accurately by Simpson’s formula by planimetering the cavities in two planes at right angles to each other. All measurements are plotted against gestational age as estimated from an early measurement of the biparietal diameter or femur length. Ratios such as the A/PA ratio or LV/RV ratio are often more useful than absolute values. Use of Color-Flow Mapping and Pulsed Doppler Color-flow mapping is an important adjunct to crosssectional scanning and can considerably speed up the cardiac evaluation. The correct direction of flow

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Fig. 14. The aortic diameter at the level of the valve is plotted against gestational age.

Fig. 15. The pulmonary diameter at the level of the valve is plotted against gestational age.

throughout the cardiac chambers and arch vessels can be demonstrated. It can be used to exclude regurgitation at any valve. At the usual Nyquist limits, unaliased color flow throughout the heart indicates that

the flow is at normal velocities. If color shows aliasing at any point in the heart, pulsed Doppler should be used to obtain an accurate velocity. The peak velocity at the atrioventricular valves is 30–60 cm/sec and is

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Fig. 16. The Ao:PA diameters are expressed as a ratio.

Fig. 17. The circumference of the heart is compared to the circumference of the bony outline of the thorax and expressed as a ratio.

fairly constant throughout gestation. The peak velocity of flow at the arterial valves is approximately 25 cm/sec at 12 weeks, increasing to 60–100 cm/sec by

term. Turning on color flow when the ultrasound beam is perpendicular to the ventricular septum helps to exclude a significant ventricular septal defect.

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Fig. 18. The right ventricle (RV) width at its widest point, which is usually in midcavity just below the leaflets of the tricuspid valve, is plotted against gestational age.

Fig. 19. The left ventricle (LV) width at its widest point, which is usually in midcavity just below the leaflets of the mitral valve, is plotted against gestational age.

Color-flow mapping is essential to confirm a normal pulmonary venous connection. Routine use of pulsed Doppler is not necessary unless the color-flow map

dictates it, but the fetal echocardiographer should be familiar with the characteristics of normal flow profiles at each point within the heart [1]. The ultrasound

L. Allan: Technique of Fetal Echocardiography

beam must be positioned ‘‘in-line’’ with the flow under observation when using pulsed or color-flow mapping. This may require some technical skill. Use of M-Mode Echocardiography This modality is seldom used in fetal assessment and is not necessary during normal heart scanning. It can be used to assess ventricular function in the rare case in which function is abnormal and it is useful in the evaluation of arrhythmias. Summary The fetal heart can be readily assessed during routine evaluation of the anatomy of the fetus by the obstetric sonographer or in high-risk patients by a specialist fetal echocardiographer. With an ideal fetal position and the transducer in skilled hands, this can literally take seconds. It involves sweeping the transducer beam up the front of the chest to display normal cardiac structures in two-dimensional imaging and repeating the process in color. A thorough familiarity with the features of the normal heart is essential in order to detect abnormalities.

Appendix Clip 1. The ultrasound beam is swept up from the abdomen to the four-chamber view. The stomach and apex are on the same side of the fetus. The correct relationship of the aorta and inferior vena cava in the abdomen can be noted. The apical four-chamber view imaged here allows all the features that need to be checked in the normal four-chamber view to be seen. Clip 2. The four-chamber view is seen in a lateral projection in which the fetus lies with the right anterior chest closest to the transducer. The ultrasound beam is perpendicular to the ventricular septum. All the normal features of a four-chamber view can be checked in this orientation, although the appearance of the four-chamber view is slightly different from the apical projection. Clip 3. The ultrasound beam is swept up the front of the chest from the four-chamber view to image the left and right outflow tracts and transverse arch sequentially. The size and positional relationships of the great arteries can be checked. The normal crossover of the pulmonary artery over the aorta is demonstrated. Clip 4. The transverse view of the duct and transverse arch are seen simultaneously on color-flow mapping. The longer vessel arising from near the front of the

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chest and directed straight back toward the spine is the pulmonary artery with its continuation as the arterial duct. The aorta arises in the middle of the thorax, forms the transverse arch on the right side of the pulmonary artery and duct, and crosses the midline to join the duct just to the left of the spine. In the normal fetus as shown here, these vessels should be of similar size and flow should be in the same direction within them. Clip 5. The ductal and aortic arches are seen in the long axis of the fetus in slightly different planes of section. The transducer beam can be swept from the duct at the start of the clip to the aortic arch at the end. The normal size and positional arrangements of the great arteries can be noted.

References 1. Allan LD (2000) The normal fetal heart. In: Allan LD, Hornberger L, Sharland G (Eds.), Fetal Cardiology. Greenwich Medical, London, pp 45–60 2. Allan LD, Crawford DC, Chita SK, Tynan MJ (1986) Prenatal screening for congenital heart disease. Br Med J 292:1717–1719 3. Allan LD, Santos R, Pexieder T (1997) Anatomical and echocardiographic correlates of normal cardiac morphology in the late first trimester fetus. Heart 77:68–72 4. Allan LD, Tynan MJ, Campbell S, Wilkinson J, Anderson RH (1980) Echocardiographic and anatomical correlates in the fetus. Br Heart J 44:444–451 5. Cordes TM, O’Leary PW, Seward JB, Hagler DJ (1994) Distinguishing right from left: a standardized technique for fetal echocardiography. J Am Soc Echocardiogr 7:47–53 6. Huggon IC, Ghi T, Cook AC, et al. (2002) Fetal cardiac abnormalities identified prior to 14 weeks gestation. Ultrasound Obstet Gynecol 20:22–27 7. Kleinman CS, Hobbins JC, Jaffe CC, et al. (1980) Echocardiographic studies of the human fetus: prenatal diagnosis of congenital heart disease and cardiac dysrhythmias. Pediatrics 65:1059–1064 8. Lange LW, Sahn DJ, Allen HD, et al. (1980) Qualitative realtime cross-sectional echocardiographic imaging of the human fetus during the second half of pregnancy. Circulation 62:799– 806 9. Sahn DJ, Lange LW, Allen HD, et al. (1980) Quantitative realtime cross-sectional echocardiography in the developing human fetus and newborn. Circulation 62:588 10. Sharland GK, Allan LD (1992) Normal fetal cardiac measurements derived by cross-sectional echocardiography. Ultrasound Obstet Gynecol 2:175–181 11. Tan J, Silverman NH, Hoffman JIE, Villegas M, Schmidt KG (1992) Cardiac dimensions determined by cross-sectional echocardiography in the normal human fetus from 18 weeks to term. Am J Cardiol 70:1459–1467 12. Yoo SJ, Lee YH, Kim ES, et al. (1997) Three-vessel view of the fetal upper mediastinum: an easy means of detecting abnormalities of the ventricular outflow tracts, and great arteries during obstetric screening. Ultrasound Obstet Gynecol 9:173– 182