"On the 21st day, of the 21st year, of the 21st Century, Noah Jon Weatherspoon greeted us! We are truly grateful," Rowland write on Instagram on Saturday.
The post, which got nearly 500,000 "likes" the first hour it was online, included a photo of her 6-year-old son Titan smiling down at his baby brother, who is lying on a bed.
Both children are wearing pajamas.
Rowland married her manager, Tim Weatherspoon, in 2014.
