Heigl, 37, and her musician husband Josh Kelley, 36, shared the news in a joint statement to People on Thursday.

"The Kelley clan is thrilled to announce that we are expecting a third addition to our family," the couple said in a statement.

The couple already have two daughters, Naleigh Moon, 7, and Adalaide Marie Hope, 4, who they said "could not be more excited to welcome their new sibling into the fold".

Naleigh was adopted from South Korea, the birthplace of Heigl's own adoptive sister, in 2009 where she was born with a heart defect and underwent open heart surgery before being flown to the US. Adalaide was adopted three years later from within the US.

Speaking about her decision to adopt Naleigh in a 2011 interview, Heigl said she had always intended to adopt.

"My older sister is Korean and she's always been a part of my life. I always knew that would be a part of what my family was going to look like," she said.

In a 2012 interview with Parent & Child, Heigl said the couple had "talked about" having biological children as well, but decided to adopt first.

"I just wanted to be a mom. So however we do that, it's fine with me," she said at the time.

The actress, who is due to star alongside Laverne Cox in the upcoming television show Doubt, told People she "couldn't be more excited" to share the news of her first pregnancy.

"This is an incredibly exciting time for us filled with hope, anticipation, and hormones. Well, only I'm filled with hormones, but everyone in the family gets to enjoy them," she joked.

Writing on her personal blog Those Heavenly Days, which documents the family's home life on their ranch in Utah, Heigl said the family had been wanting a new addition "for awhile [sic]" and "frankly didn't really care" how the new baby came about.

"We were considering adoption again, fostering, or pregnancy if possible," she said.

"Seeing as I have never been pregnant and as my OB/GYN reminded me last year, I'm of advanced maternal age, I wasn't sure pregnancy was even in the cards for us. Turns out it was very much in the cards!"