Being able to confirm normality of the fetal heart is as important as making the diagnosis of CHD, simple or complex. For the pregnant woman who is aware of the increased risk of cardiac malformation in her unborn child, reassurance is of paramount importance. For those referred because of a suspected abnormality, accuracy of diagnosis forms the platform for subsequent pregnancy management. Neither scenario can be underestimated. In both instances, it is important to approach the fetal heart in a logical manner. The SSA offers a step-by-step approach to describing the cardiac anatomy in normal and malformed fetal hearts. Determination of situs, cardiac connections and associated defects facilitates understanding of the pathophysiology of abnormalities, which is essential for counselling families.

When applied to the fetus, the SSA differs, in that prior to ascertaining abdominal situs and by inference, atrial arrangement, it is imperative to determine fetal laterality [69, 77]. This is achieved by assessing fetal lie within the maternal abdomen so that the right and left sides of the fetus can be established. Subsequently, the same 7postnatal rules and definitions used in the SSA apply.

The diagnosis of major CHD involving abnormalities of cardiac connections is often straightforward, such as in tricuspid atresia and complete transposition of the great arteries (Fig. 1.2). More complex lesions, including those seen in the setting of atrial isomerism, can also be identified accurately but are also more challenging. It can be more difficult to exclude relatively minor defects, e.g. a small to moderate perimembranous ventricular septal defect, than to diagnose a complex abnormality. If images are suboptimal, additional scans may be needed.

An important consideration in fetal heart disease relates to potential progression of obstructive lesions as pregnancy advances [78–80]. A classical example is seen in critical aortic stenosis in mid-pregnancy that is likely to progress to hypoplastic left heart syndrome (Fig. 1.3). Also to be taken into account in predicting postnatal presentation of CHD are the physiological perinatal circulatory changes. In addition to closure of the foramen ovale, ductus arteriosus and ductus venosus, changes in right and left ventricular preload and afterload may alter the appearances of the normal and abnormal heart. This is particularly relevant in cases of borderline left ventricle when trying to predict if it will be able to sustain a biventricular circulation after birth.

Fetal cardiac rhythm should be regular and heart rate roughly ranges from 120 to 160 beats per minute (bpm). M-mode echocardiography and pulsed-wave Doppler are the most commonly used methods to assess rhythm in the fetus, based on simultaneous recording of atrial and ventricular activities [81]. The most common rhythm disturbances are intermittent extrasystoles, which are of little clinical relevance. Arrhythmias that potentially affect fetal well-being or have postnatal implications for the newborn and child are relatively uncommon. Less than 10 % of referrals are due to sustained tachy- or bradyarrhythmias [81].

Intermittent extrasystoles are frequently encountered, usually of atrial origin and generally considered to be ‘benign’. They are often described as ‘skipped’ or ‘missed’ beats, which can cause a lot of anxiety, even if the vast majority resolve spontaneously and require no treatment. In about 2 % of cases, skipped beats may represent incomplete heart block [82] or be associated with intermittent tachycardia. Therefore, it is important that these possibilities be excluded. A scan performed locally by the sonographer or obstetrician usually suffices. If heart rate is within normal range, with no fluid accumulation in any fetal compartment and the cardiac screening views are normal, the pregnant woman can be reassured. Urgent referral to a specialist is only warranted in a few selected cases when either heart rate and/or the scan findings are abnormal [83]. If the ectopic beats occur ‘very frequently’ (i.e. the rhythm is irregular most of the time), there is a slightly higher risk of fetus developing a tachyarrhythmia [84] and a referral is also warranted, after the initial assessment in the local hospital.

Tachycardia is defined as rate ≥180 bpm. If persistent, it may lead to congestive heart failure and hydrops fetalis (Fig. 1.3). If it is intermittent or not, urgent referral to the fetal cardiologist is required. However, not all cases need treatment as sometimes the arrhythmia resolves spontaneously. Thus, if the fetus is stable, close monitoring of fetal heart rate for 24 h or so to determine if the arrhythmia persists may be appropriate, before initiating therapy. If there is sustained tachycardia or it persists for >50 % of the time, or the fetus is compromised, fetal treatment or delivery of the baby (if gestational age ≥37 weeks) is indicated. Choice of medication varies from centre to centre and with experience. Maternal transplacental transfer is the preferred option to deliver the chosen drug to the fetus. Currently, a randomized controlled trial for treatment of fetal tachyarrhythmia is on its early implementation phase [85].

Traditionally, fetal bradycardia has been defined as rates <100 bpm but current obstetric threshold is 110 bpm [86]. More recently, gestational age-specific heart rates have been developed and centile charts are available [87]. Transient periods of sinus bradycardia during scanning are common and benign. Persistent sinus bradycardia is relatively rare and may be a manifestation of long QT syndrome [87]. More commonly however, fetal bradycardia is due to blocked atrial bigeminy, which typically presents with heart rate around 70–80 bpm. This can be transient or last for days or weeks. Whilst it is well tolerated by the fetus and of no hemodynamic consequence, it is important to differentiate blocked bigeminy from second-degree atrioventricular block with 2:1 conduction [82]. Heart block is often caused by transplacental passage of circulating maternal IgG antibodies (anti-Ro/ SSA and anti-La/SSB), which causes injury to the conduction tissue with subsequent fibrous replacement. Certain forms of CHD can also lead to fetal heart block, notably cases which are associated with left isomerism, but it can also occur in the presence of atrioventricular discordance. The prognosis for autoimmune-mediated complete atrioventricular block is better than if associated with CHD but there still is significant mortality and morbidity. Most survivors require pacemaker implantation in the first year of life [88].

Despite recent advances in genetics and better understanding of inherited cardiac conditions that may confer a 50 % risk to the fetus, little has been reported in fetal life. Inherited cardiomyopathies are relatively uncommon in the fetus [89]. Hypertrophic cardiomyopathy rarely manifests prenatally, but can be associated with Noonan syndrome. Among channelopathies, recent data on prenatal manifestation of long QT syndrome, bradycardia with rates below the 3rd centile for gestational age, has increased interest in identifying affected fetuses [87]. Among aortopathies and related conditions, there are scarce fetal reports, mainly related to the infantile type of Marfan syndrome [90].

Initial observations of CHD diagnosed in the first trimester of pregnancy were made by obstetricians utilizing a transvaginal approach [91, 92]. Subsequently, it became clear that the transabdominal route could also be used in clinical practice to image the fetal heart at less than 14 weeks gestation [93]. Over the years, this practice has become more common. The number of fetal cardiologists offering a detailed assessment of the fetal heart at 15–16 weeks has increased but it is not yet universally available. Early scans can be challenging due to the small fetal heart size and additional technical limitations sometimes imposed by fetal position and maternal characteristics. Nevertheless, its clinical utility in high-risk pregnancies has been shown by a number of investigators [94–97]. Similarly to mid-gestation, it is very important that the fetus with CHD identified early in pregnancy be assessed by a multidisciplinary team, especially in cases referred because of increased NT measurements. Figure 1.3 illustrates a case of hypoplastic left heart syndrome diagnosed at 14 weeks in a woman referred with family history of CHD.