Giuliana Rancic shares an adorable picture of Bill and baby Duke, 9 months, in swim trunks on vacation in Mexico. Credit: courtesy of Giuliana Rancic

Baby Duke is growing up so fast! Giuliana Rancic and Bill Rancic's adorable baby boy turned 9 months old on Wednesday, May 29, and the family celebrated the milestone while on vacation at Rosewood Mayakoba in Riviera Maya, Mexico.

"Best family vacay ever at Rosewood Mayakoba with my two favorite guys . . . Bill Rancic and baby Duke." Giuliana, 38, tweeted. "Mexico. Sun and fun." In a sweet father-son picture, Bill, 42, holds baby Duke on his lap, and the two smile at the camera while posing shirtless in swim trunks.

"Best nine months of my life," Giuliana wrote on Duke's special day. "Baby Duke. Big boy. My bambino. Blessing." The proud mother also shared a photo of their mini celebration, complete with cake and balloons.

The couple welcomed their first child in August 2012 via surrogate after multiple failed IVF attempts, and Giuliana's battle with breast cancer. "I used to say, 'Oh my God, I want four kids, five kids,' but gosh one is a lot . . . one's a lot of work," Giuliana told Us Weekly in March.

But they aren't ruling out the possibility of giving Duke a sibling. "We'd love to expand our family but as of now there's nothing in the works," Giuliana said. "I just want to make sure [I'm] giving enough attention . . . not spreading [myself] too thin. I just want to make sure I'm enjoying this time with him and not rushing anything."