Katherine Heigl has announced her first pregnancy. She and husband Josh Kelley will welcome their first son in January. The tiny tot  will be the couple’s third child.

“It was totally unexpected but thrilling,” Heigl, 37, wrote in a Thursday post after sharing her exciting news exclusively with PEOPLE. “The whole pregnancy thing is new to me but the thrill of having a baby is not.”

The couple are already parents to two girls – Naleigh Moon, 7, and Adalaide Marie Hope, 4 — whom the couple adopted in 2009 and 2012.