Katherine Heigl left her scrubs-wearing “Grey’s Anatomy” character Izzy in the dust at the glitzy premiere of her new comedy, “Knocked Up.”

The star donned a knockout low-cut dress for her big night, but in the latest comedy from director Judd Apatow, she traded in everything for maternity clothes!

Katherine plays a woman who gets pregnant after a one-night stand with “The 40-Year-Old Virgin’s” loveable oaf Seth Rogen.

It’s a special movie for Katherine because she got engaged to musician Josh Kelley – and all that real-life baby talk is making Josh squirm!

Our Dayna Devon asked Josh if he knows when he and his fiancée will hear the pitter-patter of little feet – and he responded with, “Uh, man, I don’t know, I don’t know!”

It was definitely love at first sight for Josh and Katherine.

The adorable couple met on the set of his music video, “Only You,” in 2005.

“He actually proposed at the place we met,” Katherine explained. “We met doing his music video and it was on Venice Beach.”

She added, “I was actually trying to keep it like, sort of just between us – but he spilled it to all the press!”

Oops!

But Josh insisted, “Well I didn’t know I wasn’t supposed to say anything!”

Katherine wasn’t the only young TV doctor in love on the red carpet.

Her newly-engaged “Grey’s” co-star Kate Walsh arrived with fiancé Alex Young, and a solo Sara Ramirez rushed in to catch Katherine’s big-screen performance!

“Knocked Up” is due in theaters June 1.

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