She was the daughter and sole heiress of John Beaufort, Duke of Somerset (1404–1444), who was a great-grandson of King Edward III through his third surviving son, John of Gaunt by Katherine Swynford. Margaret was born at Bletsoe Castle, Bedfordshire, on either 31 May 1441 or, more likely, on 31 May 1443. The day and month are not disputed, as she required Westminster Abbey to celebrate her birthday on 31 May. The year of her birth is more uncertain. William Dugdale, the 17th-century antiquary, suggested that she may have been born in 1441, based on evidence of inquisitions post mortem taken after the death of her father. Dugdale has been followed by a number of Margaret's biographers; however, it is more likely that she was born in 1443, as in May 1443 her father had negotiated with the king concerning the wardship of his unborn child should he die on campaign.