Make way for the eleventh member of the Hanson clan: Zac, the youngest of the Hanson trio, and his wife Kate are expecting their first child in May, they tell PEOPLE exclusively.

"I am utterly thrilled at the thought of becoming a father," says Zac, 22. "I can't wait to meet this person." Adds Kate, 23, "This is the most amazing time for us. It's the most romantic thing we've ever done."

The newest addition to the Hanson family romper room will join Ezra, 5, Penelope, 2, River, 1 (middle brother Taylor, 24 and his wife Natalie's children) and Everett, 7 months (eldest brother Isaac, 27, and his wife Nikki's son). "It will be really fun to see five little ones grow up together," says Zac. "I know watching them is going to inspire a lot of music."

Not to mention a new logistics plan for the band, currently touring with their families in tow to promote their album The Walk. On a single tour bus. Says Kate, "We may have to start thi nking about expanding to two buses!"