The Word 'Begotten'
...the word begotten, when used of the divine Son, cannot be understood in terms of human begetting, most obviously because, although he had an earthly birth and a human mother, he also existed before his human birth and incarnation, yet he had no divine mother. The early theologians settled on the term "begotten" (gennao) to speak of the eternal generation of the Son because they found it repeatedly in Scripture...
Kevin Giles, The Eternal Generation of the Son: Maintaining Orthodoxy in Trinitarian Theology (Downers Grove, IL: InterVarsity Press, 2012), 3