Zac Hanson and Wife Kate Welcome Baby No. 5

Zac Hanson is officially a father of five, and we officially feel old.

The youngest member of '90s pop rock group Hanson recently welcomed a baby boy with wife Kate Hanson. Zac, 35, shared the news to Instagram on Tuesday, March 9.

"I am a little late, but I celebrated #internationalwomensday the best way I know how," he captioned black and white snapshots of the pair holding their newborn. "Marveling at the superpower women are uniquely gifted... as well as freaking out over the fact that I am now responsible to help raise another person."

So what did Zac and Kate name their fifth little one? Quincy Joseph Thoreau Hanson—no doubt quite a title to live up to!

Baby Quincy joins older siblings Mary Lucille Diana, 4, George Abraham Walker, 7, Junia Rosa Ruth, 10, and John Ira Shepherd, 12.

Zac and Kate, who wed in 2006, first announced their family was expanding in January.

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There are 15 Hanson kids among brothers and bandmates Zac, Taylor Hanson and Isaac Hanson. Last December, Taylor and wife Natalie welcomed their seventh (a little girl named Maybellene Alma), while Isaac has two sons and a daughter of his own with wife Nicole.

And nearly three decades after the brothers first formed Hanson and found overnight success with "MMMBop," their fanbase is still just as passionate thanks to a loyal online fan club.

As Zac described on E!'s Daily Pop, "You're building a relationship, you're building these experiences together. Hanson.net, our website, and the online membership community has been going for over 20 years."

For more from the guys of Hanson, including how they really feel about performing "MMMBop" after all these years, read our exclusive interview here.