Nicole Richie threw a star-studded birthday bash for her son's birthday on Saturday (07.09.13).

The House of Harlow designer, 31, was joined by a string of her close friends including actress Jessica Alba, her husband Cash Warren and their daughters Honor, five, and Haven, two, as well as pregnant singer Gwen Stefani and her sons Kingston, seven, and Zuma, four, and 'Keeping Up with the Kardashians' star Kourtney Kardashian, who brought her son Mason, three, and daughter Penelope, 14 months, at a hotel in Beverly Hills to celebrate her son Sparrow Madden's fourth birthday.

Nicole's rocker husband Joel Madden was on hand to greet their celebrity guests at the party, according to X17Online.com.

No Doubt star Gwen concealed her growing baby bump in a monochrome jumpsuit.

The 43-year-old singer and her husband Gavin Rossdale have yet to officially confirm her pregnancy but the couple are said to be delighted with the happy news and Gwen has already started nesting in preparation for the tot.

A source recently said: ''Gwen and Gavin couldn't be happier. She's just trying to focus on resting right now. She wants to make sure that all is well with the baby.''

Gwen previously admitted she wanted a third, and revealed she and Gavin, 47, tried for a baby in 2010.

She said: ''I really, really, really wanted one. It didn't really work out. So I feel good with what we've got. Everything works out how it should. You can't plan everything, right? You can try.''