Heidi Montag shares childhood photos with her brother. Credit: Courtesy of Heidi Pratt; Ethan Miller/Getty (inset)

Who's that girl? Even the biggest Heidi Montag fans might have been confused when the former reality star, 27, posted three childhood photos via Instagram in honor of her younger brother Sky's 21st birthday, featuring the star looking completely different from her present-day self. 

"Happy Birthday little Sky Sky!" Montag captioned the throwback picture of her younger self hugging her sibling and grinning to show off her braces. 

She shared several images from back in the day, each featuring the former Hills star looking more unrecognizable than the next.

Montag appeared on all six seasons of The Hills, and though Sky never made an appearance, Heidi was occasionally joined by her older sister Holly Montag

In 2010, the wife of Spencer Pratt famously underwent 10 plastic surgery procedures in a single day, which was covered on the MTV hit when her mother had an emotional reaction to the star's shocking physical transformation. 

But since then, Montag has toned down her look, and even removed her F-cup implants in favor of C-cups. 

"I couldn't conceptualize the weight of them in my body," she told Us Weekly in November 2013 of her implants. "They felt like bowling balls on my chest."