Born near Settimo Pavese, Bianca Maria was the illegitimate daughter of Filippo Maria Visconti, Duke of Milan and last of the Visconti rulers, and Agnese del Maino. Agnese was the daughter of Ambrogio del Maino, a Milanese nobleman and ducal questore. Agnese served as Lady-in-Waiting to Filippo's wife, Beatrice di Tenda. The couple had a second daughter, called Caterina Maria or Lucia Maria, also born in Settimo in 1426, but she died shortly after her birth.
