The celebrity couple - who already have a three-year-old son called Camden and a daughter called Brooklyn, 19 months - will welcome another child next year.
Vanessa has revealed that their third child will be a boy.
She shared a video on Instagram, taken earlier this year, where she revealed the news to Nick and captioned it: "I have so much to be Thankful for this year... Especially our littlest Lachey.
"When Daddy came home from tour this Summer and we were moving into our new house I shared the news! Now I'm sharing it with you! I am so very Thankful and Grateful for this family of mine! And Thankful for YOU! All your Love & support makes what we do FUN and worth it! Happy Thanksgiving Everyone! Love, V (sic).