A good mom bag should be a master of disguise, as far as Stella McCartney is concerned. "It should look like a handbag but have all the features of a baby bag," says the designer and mother of three. To get what she wanted, McCartney came up with her own—and then some. Collaborating with LeSportsac, she created a highly modular 17-piece line that runs from a makeup case to a 26-inch suitcase and is geared toward on-the-go moms.

The whole collection—including the Mum, which comes with a changing pad and insulated bottle pockets and is made from 100 percent-recycled materials, is true to both McCartney's ethics (she's an animal-rights activist) and to LeSportsac's tradition of soft, durable nylon. "We wanted to keep it eco and high fashion, which is Stella," explains LeSportsac creative director Elizabeth Kiester. "So we weren't going to go with a rough-hewn hemp."

The collection also has a little something for the little ones: a bunny-shaped knapsack and a small carry-on wheelie. Should your tot tire of pulling the latter through the terminal, it will strap onto your larger bag. (Not that you want to master disguising yourself as a skycap.)