This will be the first child for Paquin, who is due in the fall. Moyer has two children from previous relationships.
Paquin, 29, and Moyer, 42, started dating after meeting on the HBO vampire drama in 2008. They went public with their relationship in February 2009 and announced their engagement six months later. The couple wed in 2010.
True Blood returns for Season 5 on Sunday, June 10.