Credit: Beiny/Slater/WENN.com; Norman Scott/Startraksphoto.com, Clasos.com/Splash News Online

Cleavage coming out party!

When Nicole Richie donned a sexy blue bikini during a 30th birthday vacation in Cabo San Lucas, Mexico Sept. 24, she proudly flaunted her new breast implants, acquired over the summer.

But the House of Harlow 1960 designer was a bit more demure about her surgically enhanced chest before the momentous Cabo trip.

Shopping with hubby Joel Madden in West Hollywood on Sept. 13, the Fashion Star mentor flashed a bit of cleavage in a black top accessorized with short shorts and a hat. Lunching with friends at Cecconi's in L.A. on Sept. 21 (her actual birthday), the famously boho star covered up in a billowy yellow dress.

While Richie has yet to address the surgery in public or on her Twitter account, "She's been open about it to friends," one pal tells the new Us Weekly.

A second friend explained that the former Simple Life star went under the knife for the benefit of hubby Madden, father to her daughter Harlow, 3, and son Sparrow, 2.

"She said breast-feeding killed what boobs she did have!" the friend explains.

Tell Us: Do you like Richie's new figure?