Just imaging Prompto trying to hide his pregnant from Gladio as long as he can, worried about what the reaction would be. Ignis finds out first, but Prompto tries so hard to get him not to tell. And then one day, Gladio walks in and sees Prompto's belly, and the poor omega can't hide anymore and he's freaked out, trying not to cry
Things I like in this scenario: That Iggy knows first because of course he would. And maybe pregnancy wouldn’t be the first leap, but he’d notice that Prompto seems tired more often, is suddenly more picky about his food, claims to be sore or not feel so hot a lot. He might jump to illness and confront Prompto, assuming the worse and wanting to let the omega know that he doesn’t have to hide things from his friends, and there’s not much for Prompto to do but spill (he can’t have Ignis thinking he’s hiding some sort of life threatening illness when really he and Gladio are just failures at using condoms.) And Ignis won’t want to agree to keep this a secret but he’ll agree with some conditions like: Prompto has to tell at least Gladio soon. He had to see a doctor. And he has to turn himself over to Ignis’ tender, type A, anal retentive mercies (I hope you like meals tailored to omega pregnancies and early morning yoga Prompto…)
The angst potential of Prompto trying to hide things from Gladio. Breaking dates and not sleeping over anymore because he’s always sick and can’t eat anything but saltines and he doesn’t want Gladio to see that. Shying away from touch and sex, especially as he starts to gain weight, and never letting himself be seen without a shirt. At first Gladio thinks it’s just an insecurity thing, especially when he notices his lover putting on a little weight and eating differently, but there are other things: he’s spending more time with Ignis, a lot more time. Early mornings and shopping trips that go on for hours but don’t seem to actually end with anything being bought (Prompto is low key hiding baby stuff at Ignis’ place.) Temper flares and getting snappy if Gladio tries to touch him…
And finally the angst when it call comes out and Gladio is left with the reality that Prompto had been lying to him, Ignis has been lying to him, Prompto is however many months pregnant and he’s had no clue, and he only knows now because he’s figured it out himself.