Gwen Stefani has sent birthday wishes to her 13-year-old son Zuma via social media.

Zuma (c) Instagram

Zuma (c) Instagram

The 51-year-old singer took to Instagram to mark her son's birthday, posting a baby photo and an up-to-date snap of her son on the platform.

Alongside the snaps, Gwen - who also has Kingston, 15, and Apollo, seven, with ex-husband Gavin Rossdale - wrote: "happy 13th b day to my Zuma baby [baby emoji] we love u so much!! gx (sic)"

The blonde beauty also posted a snap of all three of her kids together in a family photo.

Below the group shot, she wrote: "Happy Birthday!"

Gwen posted another throwback photo of Zuma as a toddler at one of his early birthdays.

In the photo, he's seen swinging a bat at a Spider-Man pinata.

She captioned the picture: "Babies grow up."

Gwen married her long-term partner Blake Shelton in July, and he recently revealed that she had a "hard time" trying to get through her wedding vows without crying.

The pair chose to write their own vows for the nuptials, and Blake admitted that Gwen’s words were so moving they both had difficulty getting through the emotional moment without bursting into tears.

He said: "Gwen, right up until the night before, like when it started getting, like, two weeks away, she was like, 'Oh my god, let's just not do that. Let's just, like, have the repeat after me stuff.' ... And Carson [Daly, who officiate the ceremony] was adamant about, 'No, you can, look, you're a songwriter, you can do this.' And so, right up until the night before, she was working on hers and it was incredible.

“I still haven't even seen a video of our wedding yet so I can't remember all the things she said, but it was, she had them written down and she had a hard time getting through them. I had a hard time, you know, trying to, you know, get through what she said also."