Accurate visualization of embryonic vasculature provides critical insights into cardiovascular development and the origins of congenital heart defects. Here, we describe a protocol for a low-viscosity plastic resin injection into the fetal mouse heart, which then polymerizes to create stable casts of the great vessels. These casts preserve fine vascular architecture and enable the detection of structural abnormalities, such as aortic arch artery anomalies and ventricular septal defects. This method provides a robust and reproducible approach for investigating both normal and pathological cardiovascular networks in rodent embryos.