(Cover) - EN Showbiz - Sienna Miller is thrilled to be expecting her first child but has vowed not to reveal too much about her pregnancy.

The Alfie star has discussed impending motherhood for the first time in an interview with UK Vogue magazine.

Sienna posed for a shoot in the April issue, explaining she feels far enough along to talk about having a baby.

"I'm in my second trimester, so I'm I-can-talk-about-it pregnant, 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," she told the magazine.

She added that she is "feeling fine" and is looking forward to welcoming her first child with her boyfriend Tom Sturridge.

"It's all progressing nicely and it's very exciting," she said.

The actress is pictured in the publication wearing a plunging pink dress and holding a wreath of flowers which spell her name.

She also spoke about turning 30 last December. The star "didn't have the freak out which I thought I would have" because she was the youngest of her friends. "So that lessoned the blow. It's nice to start a new decade," she added.

Sienna and Tom are reportedly engaged. They started dating in mid 2011 following her split from fianc Jude Law.

Sienna has done her best to keep her private life under wraps over the last few years. She was awarded 100,000 in compensation after former UK newspaper News of the World accepted unconditional liability for her phone-hacking claims. Sienna alleges that her phone calls were being listened to and her email account was hacked. As stories continued to surface about her, the actress began to suspect someone close to her was leaking information.

The G.I. Joe: Rise of Cobra actress gave evidence at an enquiry into the issue where she said she felt like she was "living in a video game" when stories were being published about her private life. (C) Cover Media