Fuckkk now I just keep thinking of when someone is so pregnant their navel has no choice but to pop out.
They feel whatever's in their womb squirming around a lot recently. They see the ripples through their shirt and even their hoodies, sometimes even swaying or feeling a harsh thump that makes them clutch the sides of their belly. They think they can't take anymore when they feel a pop, and suddenly their shirt is rubbing against this hard nub and it's driving them up a wall.
When they touch it, it nearly makes their legs buckle. They gasp and grunt, forcing themself to hold their belly around it and not touch that nub that keeps twitching with each little thump and press. Yet even when they try to keep the fabric off it, the chill of the air almost makes it worse, a wet patch already soaking their underwear.
Maybe they have a partner that just watches before kneeling in front of them and kissing the taut underside of their belly. The kisses slowly trail to their belly button before they feel a testing lick and then a suck. They try to hold onto their partner by any means necessary as they cry out, not even able to touch themself.