Mill Stream is out of Swirral Edge (Hellvelyn), a dual winner, and is her second foal. Her first is Asymmetric (Showcasing), winner of the Gr.2 Richmond Stakes and third in the Gr.1 Prix Morny, a Group winner in England, a stakes winner and Group 1-placed in France and stakes-placed in the US, who is now a sire. Swirral Edge is a half-sister to Fashion Queen (Aqlaam), who won the Listed Westow Stakes, was placed in the Gr.3 Prix du Petit Couvert and is the dam of Noble Title, a Listed winner in Italy.

Second dam Pizzarra (Shamardal) was placed. She is a half-sister to Wunders Dream (Averti), winner of the Gr.2 Flying Childers Stakes, and to Grecian Dancer (Dansili), winner of the Ridgewood Pearl Stakes, then a Group 3.

Third dam Pizzicato (Statoblest).

Wunders Dream has bred stakes performers, among them Inyordreams, winner of the Listed Boadicea Stakes, and Fire Eyes, a winner at two who was Listed-placed. Grecian Dancer is the dam of Muffri'ha (Iffraaj), winner of the Gr.3 Darley Stakes and placed at Group 1 level, and the second dam of Al Mubhir, winner of the Listed Gala Stakes. This is also the family of the leading Hong Kong performers Mensa (Rudimentary) and Firebolt (Flying Spur).

Swirral Edge has an unraced 2-year-old colt, Tasalla (Mehmas).