Simplified First‐Trimester Fetal Cardiac Screening Protocol for Low‐Risk Pregnancies: Enhancing Efficiency and Accuracy

ABSTRACT

Introduction

Congenital heart defects (CHDs) are among the most prevalent congenital anomalies and are frequently detected in pregnancies classified as low‐risk, rather than in high‐risk groups. Advancements in ultrasound technology have heightened the emphasis on first‐trimester fetal cardiac screening. However, due to the small size of the fetal heart at this stage, significant challenges persist, underscoring the need for simplified and reproducible protocols that can be effectively applied on a population‐wide scale.

Methods

This paper describes a screening approach centred on the four‐chamber view (4CV) and three‐vessel tracheal view (3VTV) with colour Doppler imaging. This includes optimisation of ultrasound machine settings and the use of transabdominal and transvaginal approaches to enhance visualisation.

Results

Evidence demonstrates a CHD detection rate of 76% in low‐risk populations, with a 95% success rate in obtaining the necessary views.

Discussion and Conclusion

Operator expertise and familiarity with cardiac anomalies are critical for accurate detection. The early identification of CHDs enables informed decision‐making regarding pregnancy management. A simplified protocol for first‐trimester cardiac screening offers a promising tool for the early identification of CHDs, enabling timely referral and intervention. To optimise detection rates and clinical outcomes, continuous professional education and systematic auditing processes are indispensable for practitioners performing early fetal cardiac screening.

1. Introduction

Congenital heart defect (CHD) is a significant global health concern, affecting approximately nine out of every thousand newborns worldwide [1]. In Australia alone, around 2400 cases are reported annually, contributing to 6.9% of all infant deaths [2]. The majority of CHDs are established by the first trimester; therefore, there is a growing focus on early evaluation of the fetal heart [3, 4]. A significant proportion of cardiac abnormalities is associated with fetal aneuploidy, genetic conditions and other malformations [5, 6]. Early recognition of these abnormalities is crucial, as it supports informed decision‐making and often leads to recommendations for invasive prenatal testing rather than relying solely on cell‐free DNA screening.

Most congenital heart defects (CHD) are identified in pregnancies considered low‐risk. Although traditional maternal and familial risk factors request for early cardiac assessment, focusing screening efforts solely on these women have a diagnostic yield of only 3%–4%. In contrast, nearly 69% of CHD cases are detected in low‐risk pregnancies after abnormal findings during cardiac anatomy screening [7, 8, 9].

To increase detection rates and improve screening efficiency, it is essential to examine the fetal heart in all pregnancies, rather than limiting assessments to those classified as high risk [10]. Fetal cardiac screening in early gestation is challenging because of the small size of the fetal heart (< 7 mm) [11]. For first‐trimester cardiac examinations to be effective on a population‐wide scale, the screening protocol must be simplified, ensuring that it is practical, reproducible and capable of accurately identifying cardiac abnormalities [12].

Updated guidelines from the International Society of Ultrasound in Obstetrics and Gynaecology (ISUOG) and the American Institute of Ultrasound in Medicine (AIUM) both emphasise the importance of evaluating the four‐chamber view (4CV) and outflow tracts [13, 14]. However, there is no widely accepted international consensus on how to effectively conduct routine first‐trimester cardiac anatomy assessments in low‐risk population.

Several studies have explored a simplified protocol for cardiac screening that emphasises the 4CV and the three‐vessel tracheal view (3VTV) with colour Doppler imaging [15, 16]. Acquiring these views is no more challenging than capturing indirect markers for congenital heart defect (CHD), such as nuchal translucency (NT), tricuspid flow and ductus venosus flow [17, 18, 19]. Research has demonstrated that this approach is not only feasible with a 95% success rate in obtaining these views but also effective with a 76% detection rate of CHDs in low‐risk populations [20]. By focusing on these key cardiac views, this screening method efficiently identifies high‐risk groups with abnormal cardiac anatomy, enabling timely referral for more detailed tertiary diagnostic imaging.

This article introduces a practical approach to a simplified fetal heart screening protocol, emphasising the importance of image optimisation, and offering valuable strategies to enhance image quality and diagnostic confidence.

2. Operator Experience and Training

Cardiac screening and detection of CHDs in the first trimester depends on the sonographer's skill and understanding of cardiac defects [21]. Achieving proficiency in cardiac screening involves a significant learning curve, with detection rates and examination efficiency improving greatly with increased scan volume [22]. Sonographers with more than 2000 scans had a 54% detection rate for cardiac defects, compared with 32% for those with fewer scans [23]. The number of scans needed for proficiency varies by experience. A sonologist with over 1500 first‐trimester ultrasound examinations can complete a thorough fetal echocardiography in 94% of cases after performing 50 scans, with each taking about 9 min [24].

The learning curve in fetal echocardiography is steep. Novice sonographers, for instance, may need to perform at least 180 scans before they can reliably examine the fetal heart in most cases. This number rises to 500 scans to achieve near‐complete proficiency. As the sonographer gains experience, they become more adept at performing faster, more accurate scans, with research showing that after reaching a certain threshold, most heart images can be obtained in < 3 min [25].

In addition to technical proficiency, the operator must have a comprehensive understanding of normal fetal cardiac development and the wide range of congenital heart anomalies that can occur. This knowledge allows them to meticulously assess the four chambers of the heart, the outflow tracts, ensuring that even the smallest abnormalities are detected. The ability to recognise subtle variations in cardiac structures, such as valve function or blood flow patterns, often depends on the operator's experience in interpreting these complex images [26, 27, 28].

3. Optimal Gestational Age for First‐Trimester Cardiac Screening

The fetal heart undergoes significant development during the first trimester, and by 10 weeks of gestation, it has achieved its complete anatomical form. During this period, the heart begins to shift from a central location to its left‐sided orientation [29, 30]. By 13–14 weeks of gestation, the 4CV and outflow tracts are more consistently visible and can be successfully obtained in nearly every examination [31]. This period represents a critical window where the combination of advanced fetal development and improved imaging technology maximises diagnostic accuracy while ensuring early detection of congenital heart defects.

4. Simplified Cardiac Screening Protocol

Fetal cardiac evaluation in the first trimester adheres to the fundamental principles of second‐trimester heart screening, primarily utilising transverse planes for assessment. The abdominal situs, 4CV and 3VTV, both in 2D and with colour Doppler, form the foundation of a simplified protocol (Figure 1). Begin the screening by confirming that the axial view of the fetal chest is round in shape, with only one rib visible. Ideally, the fetal spine should be positioned at either the 6 o'clock or 12 o'clock position, although the heart can be assessed regardless of fetal orientation.

FIGURE 1.

First‐trimester fetal cardiac screening—Simplified cardiac protocol.

4.1. Abdominal Situs/Cardiac Position and Axis

The first step is to assess the cardiac position and axis. Confirm that both the heart and stomach are situated on the left side relative to the fetal orientation. The cardiac axis should be evaluated by examining the orientation of the interventricular septum (IVS) in relation to the chest. Ideally, the axis should be 45° to the left of the midline, with an acceptable variation of plus or minus 20°. An alternative method to assess cardiac axis is to draw a line from the spine posteriorly and to the mid‐chest anteriorly, which typically intersects the tricuspid valve. An abnormal axis can indicate conotruncal anomalies or complex congenital heart defects that warrant further investigation [32].

4.2. 4 Chamber View

Next, capture the 4CV by obtaining a transverse section of the fetal chest. In this view, ensure that the heart is in its normal position (situs solitus), appropriately sized and correctly oriented within the chest. Both atria and ventricles should be equal in size and contractility, and the septae (both atrial and ventricular) should be intact. The tricuspid and mitral valves should open and close together, with the septal leaflet of the tricuspid valve inserted more apically than the mitral valve indicating normal offsetting. Additionally, confirm that the crux of the heart—where the atrial septum, atrioventricular valves and IVS meet—is intact. This view detects approximately 50% of CHDs. Anomalies linked to an abnormal 4CV include atypical cardiac axis, tricuspid valve insufficiency, univentricular heart, complete AV septal defect, ventricle discordance due to valve abnormalities, and mitral or tricuspid valve atresia [33] (Figure 2).

FIGURE 2.

Cardiac defects identified on 4 chamber view.

4.3. 3 Vessel Tracheal View (3VTV)

After identifying the outflow tracts, the 3VTV should be evaluated. This view facilitates the examination of both the aortic and ductal arches and their confluence. The typical arrangement of the aorta and main pulmonary artery forms the ‘V‐sign’, on the left side of the trachea. The trachea demonstrated as a hyper‐echogenic ring surrounding a fluid filled space.

Given the small size of these vessels at this gestation, colour Doppler mapping is advisable. 2D imaging of the 3VTV is not required. It is crucial to confirm the presence of two arteries (the aorta and the main pulmonary artery) and to qualitatively assess their size ratio. Normally, the main pulmonary artery should be only slightly larger than the aorta. The arterial flow in the 3VTV should be unidirectional and free of turbulent flow. This view helps detect disproportionate artery sizes, reversed flow in one of the arteries and the presence of a single vessel. This view allows detection of abnormalities, such as tetralogy of Fallot, transposition of the great arteries, coarctation of the aorta and pulmonary atresia (Figure 3), and normal variants of vascular anatomy, that is, right sided aortic arch (RAA)—the aortic arch on the right side of the trachea, aberrant right subclavian artery (ARSA)—which courses behind the trachea [34, 35] (Figure 4).

FIGURE 3.

Cardiac defects identified on 3 vessel tracheal view.

FIGURE 4.

Normal variation of vascular anatomy noted at 3 vessel tracheal view.

5. Comparing Transabdominal and Transvaginal Approaches

The transabdominal approach is the most frequent method for early cardiac screening, particularly after 12 weeks of gestation. Improvements in ultrasound resolution and advanced imaging techniques, such as harmonic imaging and power Doppler, have enhanced the ability to visualise cardiac anatomy effectively, even in early gestation. The transabdominal route provides a broader field of view, allowing for the visualisation of the entire fetal anatomy. It is less invasive and more comfortable for the patient, making it a suitable first‐line option for routine screening. Studies have shown that transabdominal imaging can detect approximately 93.1% of cardiac anomalies. It is particularly effective when the crown‐rump length (CRL) is > 60 mm, and it can be successfully performed in most patients, especially after 13 weeks of gestation [30, 35].

The transabdominal route can be challenging in certain situations. Factors, such as high maternal body mass index (BMI), unfavourable fetal positions, placental interposition and a retroverted uterus, can hinder clear visualisation of the fetal heart. In such cases, the transvaginal approach offers a viable alternative. In cases where transabdominal imaging proves unsuccessful, there should be low threshold in using transvaginal sonography to complete the cardiac examination.

6. Optimising Machine Settings for First‐Trimester Cardiac Imaging

For optimal first‐trimester cardiac imaging, operators should start by fine‐tuning the grey‐scale settings. It is essential to use the highest appropriate transducer frequency to achieve the best possible resolution while also ensuring sufficient penetration to capture the necessary details. The heart should be magnified to occupy approximately one‐third of the display, allowing for improved visualisation.

When performing fetal cardiac scanning, capturing the fast‐moving structures with precision is crucial for an accurate diagnosis. Temporal resolution, which is the ability to distinguish between events that are closely spaced in time, is a key factor in this process. In ultrasound imaging, temporal resolution is typically determined by the frame rate, or the number of frames per second (fps) that the machine can acquire. A higher frame rate improves temporal resolution, making it easier to observe the rapid movements of the fetal heart. For optimal results, scanning frame rates should be maintained above 30 to 40 Hz, considering that fetal heart rates generally range from 120 to 160 beats per minute.

Operators can maximise the frame rate by adjusting several settings, such as decreasing the depth, focal zone placement, optimising overall gain, narrowing the sector angle/width and reducing the region of interest (ROI). While lowering line density and minimising the use of harmonic imaging can further enhance frame rates, these adjustments may affect spatial resolution. It is important to look beyond the factory presets and individually optimise machine settings. For example, line density controls in B‐mode and colour Doppler are independent. Employing a low dynamic range setting enhances image contrast by reducing the range of greys while using grey‐scale colour maps, such as sepia, can improve the visualisation of key cardiac landmarks. It is important to adjust the overall gain to ensure uniform echogenicity without introducing noise or losing detail, and to fine‐tune the time gain compensation (TGC) to balance brightness across different depths. Whilst spatial compounding (or known simply as compounding) may reduce temporal resolution, it does so without significantly affecting the frame rate.

Colour Doppler imaging is vital for assessing cardiac anatomy and diagnosing congenital heart defect. To improve frame rate and temporal resolution, a narrow colour box should be used. The pulse repetition frequency (PRF) should be optimised to prevent aliasing while ensuring high sensitivity to flow. As Doppler shifts are angle‐dependent, the PRF may need to be adjusted on the basis of the angle at which the cardiac structures are imaged. Using low colour Doppler gain can help reduce high‐amplitude echoes from surrounding tissues, and a low power output can minimise artefacts and distortions in the Doppler signal. Doppler signal processing also involves the use of a wall filter, which blocks Doppler signals below a certain threshold to prevent interference from moving tissues that could affect the evaluation of blood flow [36]. Additionally, employing a mid‐wall filter can help reduce noise from low‐frequency shifts, while a mid‐velocity colour Doppler range and mid‐to‐high colour persistence can minimise the effects of blood flow variations on the displayed Doppler signal. Power Doppler ultrasound is particularly useful for providing detailed views of blood flow in small vascular structures and for identifying cardiac septal defects. Improper settings, such as incorrect PRF, colour box size or colour gain, can lead to errors in image interpretation (Figure 5).

FIGURE 5.

Machine setting influence on image quality—Images with incorrect and optimised machine settings.

Both the AIUM and ISUOG have issued guidelines for the safe use of Doppler ultrasound during the 11 to 14‐week scan [37, 38]. The principle of ALARA (as low as reasonably achievable) should be followed so that examination times are only as long as necessary. Some ultrasound systems display the Thermal Index Soft Tissue (TIS) and Thermal Index Bone (TIB) simultaneously. It is important to identify the correct settings as the TIS is generally lower than the TIB. The AIUM recommends keeping the thermal index (TI) below 0.7, with a maximum of 1.0 only when necessary to obtain adequate images and keeping exposure times as brief as possible. If the TIB exceeds this level, reduce the power output. Ultrasound represents an additional heat source therefore in febrile patients the TIB may need to be reduced further.

7. Conclusion

This simplified protocol for first‐trimester cardiac screening offers a promising tool for the early identification of CHDs, enabling timely referral and intervention. Research indicates that this approach, particularly when enhanced with colour Doppler imaging, is both feasible and diagnostically accurate. The use of transabdominal ultrasound, complemented by transvaginal imaging when necessary, offers a robust method for detailed cardiac assessment. Ongoing training and education for operators are essential to maximise the effectiveness of early fetal heart screening. While first‐trimester screening for CHDs is invaluable, it does not replace the need for a routine heart evaluation during a targeted anatomy scan or fetal echocardiogram in the second trimester, as some CHDs are progressive and may not be detectable in the first trimester. Integrating this protocol into routine prenatal care, alongside ongoing operator training, can enhance early CHD detection and improve overall prenatal outcomes.

Author Contributions

Vanessa Pincham: conceptualization; writing – original draft; investigation; writing – review and editing; resources. Andrew McLennan: writing – review and editing. Jonathan Forsey: writing – review and editing. Ritu Mogra: conceptualization; investigation; writing – original draft; writing – review and editing; resources.

Conflicts of Interest

The authors declare no conflicts of interest.