Fetal pcom and posterior R PCA 

A fetal posterior cerebral artery PCAis a common variant in the posterior cerebral circulation, estimated to occur in 20-25% of individuals, where the PCA arises from the anterior circulation.

The posterior communicating artery (PCOM) is larger than the P1 segment of the posterior cerebral artery (PCA) and supplies the bulk of  the PCA. 

If bilateral fetal PCAs exist, the basilar artery is significantly smaller than normal. (Seen Here)