The couple, who married in 2002, already have four children after their son Viggo was born in 2008.
Their new baby will join Viggo, 9-year-old Ezra, 5-year-old River and Penelope, whose seventh birthday is tomorrow.
"Natalie and I are so anxious to meet our fifth mini-Hanson," Taylor told People. "As are the four siblings at home who are counting the days to the arrival."
Taylor's fifth child is due in late 2012.
His brother Zac Hanson got married in 2006 and has two children, while Isaac Hanson also wed in 2006 and has two sons.
Watch the video for Hanson's 'MMMBop' below: