Elfstan was born in FO 34 (SR 1454) in the Undertowers in the West-March part of the Shire. He was the first and only son of Fastred of Greenholm and Elanor Gamgee. He was born three years after his parents' marriage. He also had a younger sister named Fíriel Fairbairn. His maternal grandparents are Samwise Gamgee and Rosie Cotton.