"I'm pregnant."
"Me too!" Mirio says jokingly, stuffing a nearby couch pillow under his shirt. It's a very unsightly and misshapen baby bump, but he never backed down once a joke was in progress. It doesn't seem like his joke is going over well with you, however, because you're staring at him as though he just missed the point of something incredibly important.
It takes a few more moments of silence before Mirio lets the couch pillow drop from his shirt to the floor, his arms no longer holding it together but high in the air, in a sort of 'victory' pose.
"You're pregnant!" He exclaims excitedly, spinning around the room with the pillow he'd previously dropped on the floor, cradling it like a pillow. "You're pregnant with a baby! Our baby!"
"Well, I'm not pregnant with an alien, hopefully." There's instant relief flooding through your body at how well he's taken the news, despite the two of you being rather young and still early into your pro careers. A baby could make for a challenge, but it's not like the two of you hadn't had a few obstacles to fight through during the course of your relationship.
Mirio laughs at your joke, even if it wasn't very funny, due to how giddy he is. He had a close relationship with his own father, and to think that he'd get to form those type of bonds with his own child was an exciting thought. There's a moment of worry that passes through his mind when he wonders the type of quirk they might develop, but he decides now is a time for celebration. There was no need to worry about something he had no control over, because he was positive the future would turn out all right regardless.
"I can't believe it!" Mirio finally calms down after a few moments, when some neighbors knock on the door and ask why the two of you sound like there's a stampede of large animals running through your home. "You're pregnant! We're gonna be parents! I'll have to practice dad jokes now-"
"I'm not sure if-"
"Hi, I'm not sure if, I'm DAD!" He seems to get a jolt of joy out of saying 'I'm dad' and you're sure he's going to be riding this high for the next few days, but it was nice to finally have some good news. "How was that? Was it good? You think they'll like it?"
"They'll love it, honey."