The development of the vascular system in a fetus is a complex and intricate process that begins as early as 15 to 16 days post-conception. This process starts outside the embryo, specifically in the mesoderm of the yolk sac, chorion, and connecting stalk. Approximately two days later, the formation of blood vessels occurs within the embryo itself.

The initial formation of this system is facilitated by the small amount of yolk present in the ovum and yolk sac. Blood vessels originate from precursor cells known as hemangioblasts. These cells have the potential to develop into angioblasts, which are responsible for producing blood vessels. Angioblasts aggregate to form structures called blood islands throughout the embryonic discs. Spaces start forming within these islands, becoming the lumens or inner open spaces of the blood vessels. Some angioblasts situated around these spaces give rise to the endothelial lining of the blood vessels, while others form the tunics of larger vessels. An extensive network of blood vessels is formed throughout the embryo through continuous branching and fusion of blood islands.

Interestingly, many blood vessels follow the same guidance cues as nerves, often nestling closely to them. The local concentration of a differentiation factor known as vascular endothelial growth factor determines whether a vessel becomes an artery or a vein. By the fourth week of development, the rudimentary heart is capable of pumping blood through the developing vascular system, thereby establishing a connection between the embryo and the placenta.