Just because she's sporting a new accessory -- a growing baby bump -- doesn't mean Sienna Miller is going to ditch her signature tight minidress to attend "Twilight" star Robert Pattinson's 26th birthday party.

The expecting starlet stepped out in a body-hugging gray dress, showing off her baby bump and her killer legs.

Since a very pregnant Jessica Simpson has given birth to her baby girl, Maxwell Drew Johnson, could Miller be in the running for biggest baby bump in Tinseltown? Not quite. It appears as though the title is currently being held by Alyson Hannigan, who seems to have nabbed the self-proclaimed top honors.

"Congrats to Jessica Simpson! Does this mean I now have the biggest baby bump in Hollywood?" Alyson tweeted.

This will be the first child for Miller and her actor boyfriend, Tom Sturridge.

Check out Miller's growing baby bump below:

sienna miller baby bump

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