The Chelsea midfielder, 29, took to Instagram to post a snap of the sonogram.
The latest addition of the Fabregas household will be the youngest of three, with daughters Lia and Capri born in April 2014 and July 2015 respectively.
Fabregas also announced their third child would be a boy, in his caption on Instagram.
The Spanish star wrote: “Another baby is on the way, and this time it’s a boy.
"I couldn't be any happier. I hope everyone is feeling as blessed as I am today.
"Cheers to you my love... @daniellasemaan #GodBlessMyFamily #ThankYouGod"
Fabregas met his Semaan, 41, back in early 2013 - as she was going through a divorce to millionaire businessman Elie Taktouk.
Fabregas has been at the Stamford Bridge club since 2014, following three years at Barca and eight in north London.
He has made 100 appearances for Chelsea, netting 13 times, to go with his 105 international caps.