Let the bump watch begin: Pregnant Nicole Richie is starting to show.

The reality star, 27, showed off a baby bump Thursday night in Los Angeles as she and beau Joel Madden made the rounds at the “Eco Mom Party,” which attracted the likes of Ben Affleck, Diane Lane and Rosario Dawson.

See Nicole Richie’s baby photo

Richie and Madden, 31 - who welcomed daughter Harlow last January - announced…