William was born in 1027 or 1028 at Falaise, Duchy of Normandy, most likely towards the end of 1028. He was the only son of Robert I, son of Richard II. His mother Herleva was a daughter of Fulbert of Falaise; he may have been a tanner or embalmer. She was possibly a member of the ducal household, but did not marry Robert. She later married Herluin de Conteville, with whom she had two sons – Odo of Bayeux and Robert, Count of Mortain – and a daughter whose name is unknown. One of Herleva's brothers, Walter, became a supporter and protector of William during his minority. Robert also had a daughter, Adelaide, by another mistress.