When it comes to Hollywood, no news is more exciting than when a celeb announces she's expecting! From guessing the gender to waiting for the first pics of the bundle of joy, we can't resist following the pregnancy.

For many of these stars, the pressure is on to get back in shape after giving birth. After hitting the gym for months -- or sometimes even weeks! -- new moms love showing off the product of their hard work. Here's a look at some of the biggest transformations and as a result, unbelievably fit post-baby bodies in Hollywood.

Kim Kardashian

Larry Busacca/ Ethan Miller, Getty Images

Jessica Simpson

Alberto E. Rodriguez/ Jamie McCarthy, Getty Images

Lily Aldridge

Jason Merritt/ Jamie McCarthy, Getty Images

Jenna Dewan-Tatum

Frazer Harrison, Getty Images/ PacificCoastNews

Kristen Bell

Jason Merritt/ Christopher Polk, Getty Images

Snooki

Rob Kim/ Theo Wargo, Getty Images

Hilary Duff

Juan Sharma/Bruja, PacificCoastNews.com/ Paul A. Hebert, Getty Images

Beyonce

PacificCoastNews.com/ Larry Busacca, Getty Images

Kendra Wilkinson

Nate Jones/Sam Sharma/PacificCoastNews.com/ Frederick M. Brown, Getty Images

Amber Rose

Steve Granitz, WireImage/ Frazer Harrison, Getty Images

Fergie

Jason LaVeris, FilmMagic/ Frederick M. Brown, Getty Images

Tori Spelling

Alberto E. Rodriguez, Getty Images/ Sean O'Neill, PacificCoastNews

Penelope Cruz

SolarPix, PacificCoastNews.com/ Stuart C. Wilson, Getty Images

Reese Witherspoon

Andreas Rentz/ Toby Canham, Getty Images

Mariah Carey

Kevin Mazur, WireImage/ Optic Photos, PacificCoastNews.com

Jessica Alba

Jason Merritt/ Michael Buckner, Getty Images