Little is known for certain about Herleva's background, or the circumstances of William's birth. The written evidence dates from a generation or two later, and is not entirely consistent. Probably she was the teenage daughter of a tanner named Fulbert from the small Norman town of Falaise, where they lived. Translation being somewhat uncertain, Fulbert may instead have been a furrier, embalmer, or a person who laid out corpses for burial.
Legend has is that it all started when Robert, the young Duke of Normandy saw Herleva washing in the river near his castle. He was unable to resist her and took her for his mistress. She later gave birth to his son, William in 1027 or 1028.
Their love affair didn't last, while Robert went on a pilgrimage, Herleva married Herluin de Conteville in 1029. From this marriage she had two sons: Odo and Robert, who both became prominent during William's reign. They also had at least one daughter, who married William, lord of La Ferté-Macé.
William the Conqueror had a (half or full) sister, Adelaide, who may have been Herleva's daughter, but could possibly have been a daughter of Robert by some other mistress. Adelaide married first Enguerrand, count of Ponthieu, second Lambert of Lens, and third Odo, count of Champagne.
Herleva probably died around 1050.