Sienna Miller on February 14, 2012 and on May 24, 2012. Credit: Ben Hider/WireImage.com; BAR/Startraksphoto.com

For a first-time mom, Sienna Miller is a pro at choosing stylish maternity ensembles.

The 30-year-old actress -- who's expecting her first child with fiance Tom Sturridge -- first debuted her baby bump at W magazine's "Ever Changing Face of Beauty" exhibit in NYC on Feb. 14. Miller accentuated her tiny bump by wearing at tight, long black dress with a high, front slit.

"I'm in my second trimester, so I'm I-can-talk-about-it pregnant," the private star told the April issue of British Vogue. "Even though I'm under strict instructions not to say anything as it would completely defeat the point of everything I've tried to achieve in the last eight years."

But when's she not walking a red carpet, Miller has kept her pregnancy style comfy and casual. The mom-to-be wore a short, floral print sundress on her way to a yoga class in North London on May 24. She belted the dress low around her bump and paired the look with a glowing smile.

"I'm in my second trimester, so I'm I-can-talk-about-it pregnant," the private star told the April issue of British Vogue. "Even though I'm under strict instructions not to say anything as it would completely defeat the point of everything I've tried to achieve in the last eight years."

"I'm feeling fine," Miller said. "It's all progressing nicely and it's very exciting."