Developmental Changes of the Coronary Sinus Between the First and Second Trimesters: A Pilot Study

The morphological evolution of the fetal heart involves complex transitions in venous return pathways and chamber proportions during early gestation. Prior research has shown that the Coronary Sinus (CS) functions as the primary vessel for returning deoxygenated blood from the myocardium to the right atrium. While second-trimester cardiac anatomy is well-documented in standard obstetric practice, the specific dimensions of the Coronary Sinus (CS) during the First Trimester (FT) remain under-investigated. Clinicians often lack standardized reference ranges for the relative size of the Coronary Sinus (CS) compared to the surrounding atrial tissue in early pregnancy. Understanding these developmental shifts is necessary to distinguish between normal physiological prominence and potential markers of congenital heart defects or persistent left superior vena cava. This absence of evidence motivated the current retrospective analysis of cardiac sweeps to quantify these early developmental changes.

This investigation evaluates the presence, physical dimensions, and the Coronary Sinus to Atrial Ratio (CS/A) across the First Trimester (FT) and Second Trimester (ST) of fetal development. The researchers sought to determine if the visibility of the Coronary Sinus (CS) varies significantly as the fetus progresses through these distinct developmental stages. Another objective involved quantifying the precise relationship between the Biparietal Diameter (BPD) and the growth of specific cardiac venous structures. The study aimed to establish whether the Coronary Sinus (CS) appears disproportionately large during the First Trimester (FT) compared to the Second Trimester (ST). Investigators also examined whether maternal or fetal characteristics influence the ability of sonologists to visualize these vessels during routine ultrasound examinations. By comparing the Coronary Sinus to Atrial Ratio (CS/A) between trimesters, the team intended to provide a baseline for normal cardiac development and growth rates.

This Institutional Review Board (IRB) approved retrospective study analyzed a cohort of 99 fetuses with documented cardiac data from a single center. Inclusion criteria required the presence of adequate First Trimester (FT) cardiac sweeps and confirmed normal heart morphology during subsequent Second Trimester (ST) evaluations. A single sonologist performed all measurements of the Coronary Sinus (CS) and atrial diameters to ensure high inter-measurement consistency and minimize observer bias. The researchers utilized linear regression models to assess the specific relationship between the Biparietal Diameter (BPD) and the diameters of the Coronary Sinus (CS) and the atrium. Statistical significance for all comparisons, including the Coronary Sinus to Atrial Ratio (CS/A) differences, was defined by a P-value of less than .05. The team also collected comprehensive maternal and fetal characteristics to identify potential confounding factors that might affect the visualization of the Coronary Sinus (CS).

The Coronary Sinus to Atrial Ratio (CS/A) was significantly higher in the First Trimester (FT) at 0.43 compared to 0.25 in the Second Trimester (ST), yielding a P-value of less than .0001. Sonologists successfully visualized the Coronary Sinus (CS) in 79.2% of First Trimester (FT) fetuses, whereas it was seen in only 43.8% of Second Trimester (ST) cases. Statistical analysis revealed no significant associations between the ability to see the vessel and the maternal or fetal factors analyzed in the study. Linear regression of the combined data showed a positive correlation between Biparietal Diameter (BPD) and Coronary Sinus (CS) size with a calculated slope of 0.018. Atrial diameters also correlated positively with Biparietal Diameter (BPD), exhibiting a much steeper slope of 0.135 during the same developmental period. These findings indicate that while both structures grow, the atrium expands at a significantly faster rate than the Coronary Sinus (CS) as the fetus matures.

The Coronary Sinus (CS) appears more prominent during the First Trimester (FT), which the authors attribute to differential growth rates between the vessel and the atrium. Embryonic remnants and variations in myocardial drainage patterns likely contribute to the observed prominence of the vessel in early gestation. Hemodynamic shifts occurring between the First Trimester (FT) and Second Trimester (ST) may also play a role in the changing visibility of these cardiac structures. These results suggest that a large Coronary Sinus (CS) in the First Trimester (FT) may be a normal developmental finding rather than a pathology. The study highlights the potential utility of the Coronary Sinus to Atrial Ratio (CS/A) as a metric for assessing early fetal cardiac health. The researchers conclude that larger prospective studies are required to validate these findings and determine the clinical value of the First Trimester (FT) Coronary Sinus to Atrial Ratio (CS/A).