Amanda Eckstein and her partner Phillip Werner, from Indiana, were posing for their photographer Kristen Zaffiro of Ohio when their beloved pet horse Buckshot wanted in on the action.
And proving he too could follow instructions, when Zaffiro instructed him to smile, he sure did.
Posting the now viral photos to Facebook, Zaffiro said: "Who knew when I told THIS horse to smile, he would!"
In the series of photos, the couple at first look at each other lovingly as Amanda cradles her bump, and Buckshot stands by obediently.
But as they progress, the four hoofed family member appears to get the idea of the shoot and starts grinning too, as Amanda looks over at him seemingly confused by the behaviour. Both she and Phillip then fall into hysterics.
The couple saw the humour in the gaff, using the photos as their Facebook profile picture. Many commenters on the site couldn't get enough of Buckshot.
"Love how the horse goes from a small smile to looking like he's dying of laughter,' said one.
"I haven't laughed this much over a FB post in a long time! Need to go back and do baby photos with this horse - but a few without him too - he will steal the show from the baby!!," joked another.
"This is the cutest thing I have seen in a long time! I'm surprised momma didn't have that baby right then and there!," agreed a third.
Though for Eckstein, who is reportedly due any day now, it was another in Buckshot's long list of antics.
"Buckshot loves to be the centre of attention and being in the midst of everything," she told TODAY Parents. "Like our maternity shoot is all about him. We were trying to get photos with the rest of our horses, and he would keep walking by. He knew what he was doing."
It's not the first time the horse has gotten up to mischief on the couple's farm.
"He'll open the gates and set the other horses loose. He thinks it's funny to bust through fences. He's constantly doing silly things, like we've had horses untied in the middle of the night."