"We are over the moon with happiness and Milo is the perfect big brother," the couple told People.
Cat first announced her second pregnancy in January 2018 on Twitter.
"Over-the-moon to share that Milo is going to be a big brother! Can't wait to be a family of four in the spring. We're all so excited," Cat wrote.
At the time, Cat opened up about the joys and difficulties of being a mom.
"I think it's a hard job -- if you want to do a really good job, it's hard work. If you really care and you really want to make it, you put pressure on yourself," Cat previously told the magazine.
The longtime So You Think You Can Dance host added, "That's kind of what I think, but I don't think there's any other job in the world where you have to put love into every second of it. You can't coast."
Cat noted it's "the best job in the world" but it's "not always easy."
"The best things in life never are," however, she told People.
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In March, Cat admitted she and her husband weren't agreeing on baby names and they were "just going to wing it" when that time came.
"I think I will always win at the end of the day, because I've done all the hard work," Cat joked. "So I'm just not going to say anything. Let's not argue about it right now. I'll just win when it comes up, right?"
The British television personality wed Patrick, an Irish comedian, in a hush-hush ceremony in Rome, Italy, in October 2012.