"I'm a big brother!!" Cobb wrote on top of a photo of her firstborn with a bow tie and balloons.
The 36-year-old second-time mom-to-be has already proven she's a pro at being pregnant. In a throwback post, Cobb shared: "#tbt to last year at Comicon, 9 months pregnant supporting my nerd husband til 3am. STANDING and SOBER til 3am. If that ain't love then I don't know what is."