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No doubt about it: Gwen Stefani's kids are growing up.

The singer, 53, celebrated her son Apollo's 9th birthday on Feb. 28 with a sweet tribute on Instagram. 

"My world is so much bigger with u in it, happy birthday, Apollo," she wrote, "I love u so much." 

Along with the message, Gwen posted adorable photos and videos of her youngest son from over the years—including one of them with his stepdad Blake Shelton—set to her No Doubt song "Running." 

The "Hollaback Girl" artist shares Apollo as well as sons Kingston, 16, and Zuma, 14, with her ex-husband Gavin Rossdale. The former couple welcomed Kingston, their first child together, in 2006, and his birth was the start of a new chapter for Gwen.

"I remember thinking, 'Oh my gosh, I'm going to be a mom? Like, that's going to be weird when people tell me I'm a mom.' That word just sounded weird," she said on a March 2021 episode of DJ Khaled's podcast The First One. "But I got over that really fast."

And it's a role that the rocker has fully embraced.

"All I ever really wanted was to have kids," Gwen continued. "I have this incredible mom. And my parents are still married, and they were such an incredible example of true love and respect. And you have that growing up, it's like, ‘I want that. I want to be that. I want to be my mom when I grow up.' So to finally have kids at 36 after having this extraordinary, weird of a life that I never was expecting, it was just really incredible but really hard. And definitely the hardest thing that I've ever done in my life is having kids."

Despite the challenges that come with being a parent, the musician is thankful for her life with her family. 

"At a certain point, you have human beings that have their issues and you just have to make sure that they're happy and that they don't make big mistakes and it's a lot," Gwen added. "So, I don't take it lightly. And I'm so grateful that I have them and I got to have three boys. I don't know why god chose me to be surrounded by boys my whole life, but here I am just surrounded by them and I just feel really blessed. It's really fun."

It's fun for Blake, too, who has noted that while he takes his role of stepdad seriously he also loves just hanging out with the kids.

"I also have a blast with it," he said on a February 2021 episode of the KFROG radio show The Ride With Kimo & Heather. "I'm not gonna lie. I don't take it so serious that I'm not enjoying this time because I really am. Especially, you know, now that we're five years into this thing, I can't imagine my life without these kids now."

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