The actress beauty, 31, shared the pic while on an Arizona spa vacation with Stamos.
McHugh captioned her photo, "Good morning baby of mine. Enjoying our #babymoon at @miraval_arizona #babybump debut."
Stamos took the picture while they enjoyed the Tucson resort known for encouraging "mindfulness."
Stamos was previously married to actress Rebecca Romijn, but they didn't have kids.
Saturday was the first time McHugh had shown off her burgeoning belly on social media.
The actress has told PEOPLE, "The look on John's face when I told him we were pregnant was priceless. It was the look of a man who has wanted a family of his own but wasn't sure it was going to happen for him. Now it is!"
Stamos said that he and McHugh had previously "talked about" having a child.
The star who has been on Fuller House, the reboot of his famous sitcom, claimed that McHugh suggested they have a kid before their wedding. When he asked "why?" she reportedly said, "because you're old."
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