In a recent book called The Original Jesus (1995), Elmar Gruber and Holger Kersten argue that the virgin birth story was borrowed from a Buddhist source, which claims the same kind of origin for Buddha. This view overlooks the significant differences between the Buddhist story and the Gospel accounts. The preincarnate Buddha comes in the form of a white elephant who enters the side of his mother. Parrinder notes that it “was not a virgin birth, since she was married, and in this story … it is celestial influence rather than a divine seed that enters her.”
Most importantly, Gruber and Kersten ignore the fact that the Buddhist sources are dated long after the writing of Luke and Matthew. The story comes from a fifth-century text and is absent from the most ancient Pali canon of Buddhism. If any borrowing is going on, it is more likely that Buddhists selectively borrowed from the Gospels than vice versa. Furthermore, the oldest accounts of the life of Buddha do not depict him as a supernatural figure, but as an illuminated sage.
— Douglas Groothuis, from Jesus in An Age of Controversy, p. 218