The British fashion designer - the daughter of Beatles legend Sir Paul McCartney - delivered her fourth child with husband Alasdhair Willis on November 23 at Portland Hospital in London and the tot weighed a healthy 8lbs.
The overjoyed couple have named their new arrival Reiley, who joins her three siblings, Miller, five, Bailey, three, and two-year-old Beckett.
The 39-year-old fashionista was recently inspired by her large family to make an affordable range of clothes for children which she wants to be "timeless".
She said: "I just find it wrong to do a kids` line and be incredibly expensive, purely by dint of being a designer brand.
"Children`s clothes have become disposable in order to be accessible. I wanted to do affordable and accessible kids clothes that are still timeless with a great quality that you would want to pass down for generations.
"Kids and parents, aunts, uncles, friends, should all be able to have access to StellaMcCartney Kids clothes. We have tried to make that possible in this first collection, and hope it is enjoyed!"
The fourth addition to Stella`s family mean`s Paul McCartney now has seven grandchildren.