1.2 Fetal Risk Factors & Referral Screening

Fetal Risk Factors & Referral Screening

While maternal history provides essential clues, many cases of congenital heart disease (CHD) occur in pregnancies without any known maternal risk factors. Consequently, fetal ultrasound findings and genetic screening play a pivotal role in identifying at-risk fetuses. When an obstetrician or sonographer detects an anomaly or a suspicious finding during a routine prenatal ultrasound, a referral for a targeted fetal echocardiogram is mandated.

Increased Nuchal Translucency (NT)

Nuchal Translucency (NT) refers to the sonographic appearance of a subcutaneous accumulation of fluid behind the fetal neck in the first trimester. It is typically measured between 11 weeks and 13 weeks 6 days of gestation.

An increased NT measurement (≥ 3.5 mm or > 99th percentile) is a powerful marker for chromosomal abnormalities (such as Trisomy 21, 18, and 13). However, crucially, an elevated NT is also a major independent risk factor for major structural congenital heart disease, even in fetuses with a completely normal karyotype.

• The risk of major CHD increases exponentially as the NT measurement increases.
• For an NT between 3.5 mm and 4.4 mm, the incidence of CHD is roughly 3%. If the NT exceeds 5.5 mm, the risk of structural heart defects jumps to over 10%.
• Therefore, any fetus with a first-trimester NT of 3.5 mm or greater warrants a detailed mid-trimester fetal echocardiogram, regardless of normal non-invasive prenatal testing (NIPT) or normal amniocentesis results.

Suspected Cardiac Anomaly on Routine Screening

The most common indication for a fetal echocardiogram is the suspicion of a cardiac defect during a routine level II obstetric anatomic survey. Obstetric guidelines mandate the visualization of the four-chamber view and the left and right ventricular outflow tracts. If a sonographer notes asymmetry, abnormal axis, a missing vessel, or a septal defect, a specialized fetal echo is required to confirm the diagnosis, define the complete anatomy, and formulate a perinatal management plan.

Extracardiac Anomalies

The development of the fetal heart shares embryological pathways with several other organ systems. Consequently, the discovery of an anomaly in another organ system significantly increases the likelihood of a coexisting congenital heart defect.

Major extracardiac anomalies that necessitate a fetal echocardiogram include:

• Omphalocele: A midline abdominal wall defect. Up to 50% of fetuses with an omphalocele have concurrent CHD (often Tetralogy of Fallot or VSDs).
• Congenital Diaphragmatic Hernia (CDH): Associated with left-sided heart hypoplasia due to mediastinal shift and compression, as well as structural defects.
• Duodenal Atresia: Highly associated with Trisomy 21, thus carrying a strong correlation with atrioventricular (AV) septal defects.
• Tracheoesophageal (TE) Fistula: Often part of the VACTERL association (Vertebral, Anorectal, Cardiac, Tracheoesophageal, Renal, and Limb anomalies).
• Single Umbilical Artery (SUA): While an isolated SUA only slightly increases the risk of CHD, if it is seen alongside any other minor marker, a fetal echo is indicated.

Chromosomal and Genetic Abnormalities

Confirmed or highly suspected chromosomal anomalies require comprehensive cardiac evaluation, as many syndromes have classic cardiac manifestations:

Fetal Arrhythmias

A persistent irregular, unusually slow (bradycardia), or unusually fast (tachycardia) fetal heart rate requires immediate evaluation. A fetal echocardiogram is necessary not only to diagnose the specific type of arrhythmia (e.g., Premature Atrial Contractions, Supraventricular Tachycardia, Complete Heart Block) using M-mode and Doppler techniques but also to rule out underlying structural anomalies that may be causing the rhythm disturbance (e.g., L-TGA, Ebstein anomaly).

Hydrops Fetalis and Twin-Twin Transfusion Syndrome (TTTS)

Non-immune hydrops fetalis is defined as abnormal fluid accumulation in two or more fetal compartments (e.g., pleural effusion, pericardial effusion, ascites, skin edema). Cardiovascular failure—often due to severe structural anomalies, tachyarrhythmias, or premature ductal closure—is a leading cause of non-immune hydrops. A detailed assessment of cardiac function (Tei index, venous Doppler) is critical.

In monochorionic twin pregnancies, Twin-Twin Transfusion Syndrome (TTTS) creates a severe hemodynamic imbalance. The recipient twin often develops volume overload, leading to biventricular hypertrophy, cardiomegaly, tricuspid regurgitation, and eventually right ventricular outflow tract obstruction. Serial fetal echocardiograms are essential in staging TTTS and monitoring cardiovascular function before and after laser photocoagulation therapy.