Pregnant Sienna Miller Bares Her Bump on “Vogue” Cover — and Accidentally Reveals the Sex of Her Baby on the Way

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Miller is already mom to daughter Marlowe, 11

<p>Annie Leibovitz/Vogue</p> Sienna Miller on cover of

Annie Leibovitz/Vogue

Sienna Miller on cover of 'Vogue.'

Sienna Miller is ready to be a girl mom of two.

The pregnant actress, 41, chatted with Vogue for the cover of their December issue, opening up about her second pregnancy. In a companion video for the interview, Miller accidentally lets it slip that she's expecting a girl, who will join big sister Marlowe, 11.

Asked if Marlowe is excited to be an older sibling, Miller says, "To have a sister? A baby sister? Oh, gender reveal by accident. I'm having a baby girl."

On the cover of the magazine, the star shows off her bare bump, wearing a navy blue knit sweater that cuts off just before her stomach and black bikini bottoms.

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<p>Annie Leibovitz/Vogue</p> Sienna Miller on cover of 'Vogue.'

Annie Leibovitz/Vogue

Sienna Miller on cover of 'Vogue.'

Related: Pregnant Sienna Miller Celebrates Baby on the Way with Pink-Centric Baby Shower — See Photos!

Miller is expecting her second baby, her first with boyfriend Oli Green, 27. She shares daughter Marlowe with ex Tom Sturridge.

In August, PEOPLE confirmed the actress' pregnancy after she was photographed in a bikini with a growing bump while on vacation in Ibiza.

Throughout her pregnancy, teh Anatomy of a Scandal actress has had several high-fashion moments with her bump on display, including the Vogue World Show during London Fashion Week in September.

<p>Annie Leibovitz/Vogue</p> Sienna Miller for 'Vogue.'

Annie Leibovitz/Vogue

Sienna Miller for 'Vogue.'

She wore an epic cropped white, long-sleeve top that stopped just before her bump, baring her belly, and matched it with a voluminous white skirt. She posed on the red carpet at the event — which kicked off London Fashion Week — with her hand on her hip, accentuating her growing bump.

In November, the actress’ sister shared photos from Miller's baby shower on Instagram, writing, “Last night we showered @siennathing in love in preparation for the most exciting arrival. It was pure magic.”

She added, “I can’t wait to meet the newest member of our family 💕🌸🩷🌸💕🌸🩷🌸💕.”

The shower featured a gorgeous array of pink decor, including some impressive balloon displays, a flower crown for Miller, and several pink-centric bouquets. There was also a nail art station for guests.

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