"The day I became your mom was the day I found purpose in my life," the 46-year-old reality star wrote, alongside a heartwarming video montage of family photos of Gia at different ages. "Gia, you are such a sweet, gentle loving smart person and I’m so fortunate to be chosen as your mother."
Giudice -- who is also the mother to 15-year-old daughter Gabriella, 14-year-old daughter Milania and 10-year-old daughter Audriana -- went on to thank the birthday girl for "being such an amazing big sister, cousin, niece, friend & all the other roles you play."
"I don’t know what I’d do without you," the adoring mom continued. "You’ll never understand how much I love you & how grateful I am. I’m so proud & excited to see where your future takes you! Happy Birthday my gorgeous girl!! 18 - I can’t believe it!!!"
The sweet milestone birthday comes amid a difficult time for the family as the reality star's husband, Joe Giudice, is currently serving a 41-month prison sentence at the Federal Correctional Institution in Fort Dix, New Jersey, which began in March 2016.