The 28-year-old and her husband Ryan Reynolds are said to be "thrilled" about the news.
It is the second child for the celebrity couple – who are parents to 16-month-old daughter James.
Reynolds has previously said he would love to have a home full of daughters.
"I'm just fine being surrounded by estrogen. I grew up in a house full of testosterone. I’m the youngest of four boys, and my dad was a cop, and that’s intense," he told PEOPLE.
"For whatever reason, I always got along great with girls. I don’t have to prepare to be wrapped around my daughter’s finger – it has happened."
Rumours started about the pregnancy when the Gossip Girl star was spotted covering her belly on set.
Perez Hilton reported that Blake covered herself in a heavy robe and shirt "most of the time" on the set of her new film The Shallows.
The news comes after Reynolds joked about his sex life at the MTV Movie Awards.
While accepting the award for Best Comedic Performance he said: "I want to thank my wife—everything I do is to make her laugh, especially the sex. Thank you."
So, that funny sex is working well.
Feature Image: Getty.