"Ireland, Carmen, Rafael, Alec and I are excited to share with you that we are going to have another addition to our family," she wrote alongside a photo of her and her husband.
"A little boy coming this fall."
The couple are parents to Rafael, nine months, and Carmen, two. Alec is also dad to Ireland, 20, with his former wife Kim Basinger.
Baldwin, who is a yoga instructor, opened up about family life with two children back in 2015.
"My day never ends. It never begins and it never ends, because I'm up every two hours with him and [Rafael] sleeps in bed with us," she told E! News.
"Carmen is pretty good. She has moments where she's like the best big sister ever, where she's like taking care of him and helping bathe him and change his diaper, and then she has moments where I'm not allowed to feed him or touch him," Baldwin laughed.
"We go between the two."
Congrats to the couple!
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