The 27-year-old actress announced the news via her website, posting a picture of her growing baby bump following a family themed entry.
In the snap, the blonde actress can be seen cradling her blossoming tum as she stands in a sun-kissed field.
Lively has been married to husband Ryan Reynolds for two years, who previously wed Hollywood starlet Scarlett Johansson.
Coincidentally, Scarlett welcomed her first child with fiancé Romain Dauriac just this summer.
Announcing the news to her followers on her lifestyle site Preserve, Blake wrote: ‘The entire next month will be sprinkled with highlights for women in this exciting chapter of their lives. Congratulations to all the expecting mothers out there.’
The actress previously told Marie Claire: ‘If I could spit out a litter of kids, I would,’ whilst husband Ryan admitted he would love to have a big family with Blake.
‘We both come from big families - my parents did four, Blake’s did five. A lot of people say it’s crazy, but we’ll only know when we’re there, you know? We’ll walk through that fire pretty happily, I think.’
Congratulations Ryan and Blake!