Strawberry Marks - technically known as infantile hemangioma (IH) - are benign skin tumours with a reddish raised appearance. Although colloquially known as 'Birth Marks' they tend to appear in the first four weeks - and typically fade by the ninth year.

They occur in around 5% of babies and young children.

The cause of hemangioma is currently unknown, but several studies have suggested the importance of estrogen signaling in proliferation. Localized soft-tissue hypoxia coupled with increased circulating estrogen after birth may be the stimulus. Also, a hypothesis was presented by researchers that maternal placenta embolizes to the fetal dermis during gestation, resulting in hemangiomagenesis^. However, another group of researchers conducted genetic analyses of single-nucleotide polymorphism in hemangioma tissue compared to the mother's DNA that contradicted this hypothesis.

Source : Wikipedia