Credit: Ben Hider/

See Sienna's bump!

Sienna Miller hasn't spoken much about her first-ever pregnancy -- but she didn't' need to say a word at Tuesday night's opening reception for W magazine's "Ever Changing Face of Beauty" exhibit at the Park Avenue Armory in Manhattan.

Clad in a tight black halter dress, the British actress, 30, debuted an undeniable baby bump. The low-key star has been similarly mum about her engagement to the baby's father, fellow Brit actor Tom Sturridge. She flashed a giant diamond and platinum engagement ring while out and about in London last Friday.

"Friends are really happy with her relationship with Tom," a source told Us Weekly back in January of Sturridge, 26, her beau of over a year. "He's younger, yes, but he's mature and he's a good guy."

Miller and Sturridge's friends "were not surprised by the news of the pregnancy," says the source. "They're really good together."