The couple, who met in 2000 while presenting CBBC together, married during a trip to New York last December.
"Today is D Day," Bell told followers on her Twitter account, before confirming the news. "Just to let you know Mikey Underwood and I are having a baby! Had the scan this morning and all is well. All very exciting!"
Bell continued, joking: "Now you know why I've been talking about food non-stop for the past months. Oh and you guys can stop asking us when we going to start trying for a baby! LMAO!"
An "excited" Underwood added: "Thanks for all the lovely messages. With half of my DNA and half of Angellica Bell, I can only imagine what Baby Underwood will be like!"
Alan Carr, Jenni Falconer and Sarah Cawood are among those who have congratulated the couple on their news.
Bell recently competed in the second series of ITV's reality series 71 Degrees North.