"We couldn't be more excited," Sims wrote in a blog post. "I can't wait to meet our baby girl and I just know that Brooks is going to be the very best big brother ever. Two kids under three … here we GO!"
She also posted the news on Instagram with an adorable pic filled with pink balloons and the simple caption, "IT'S A GIRL!"
Sims and Stuber, who tied the knot in September 2011, welcomed their first child, son Brooks, in June 2012.
Recently, Sims, Vanessa Lachey, Rachel Zoe and Kerry Washington spoke with ET, where the moms and mothers-to-be shared their thoughts on pregnant supermodel Sarah Stage, who is barely showing despite being nearly nine months pregnant.