The concept of a pregnancy only progressing when you cum or when someone touches you is hot and all, but what if it was something random that made you pregnancy progress, and you're not entirely clear what that thing is?
You've been cursed by a witch to swell up with slime eggs over and over with slime eggs until she feels you've suffered enough for the wrong you've done to her. Honestly, whatever you did didn't really piss her off that much, she was just falling behind in her work and you presented the perfect opportunity to help out. She tells you your pregnancy will only progress when you do certain actions. For shits and giggles, she makes sure the criteria for progressing will be different for each clutch of slime eggs you carry, but decides not to mention that.
The first time, it's a classic every-time-you-cum situation. And oh man, are you horny as fuck the entire time. While you were quite upset at the beginning, by the time you're giving birth, you've conditioned yourself to get aroused by your condition, the sight and sensation of your belly growing after you've cum bringing you right to the edge of another orgasm. Even as the force of your orgasm pushes the first egg out of your body, you can feel their siblings still growing bigger inside of you.
The next time, you're extremely disappointed when you spend an entire day masturbating and your belly is still only a little bump. It turns out this time that your pregnancy only progresses when someone else cums inside (or even on) you. You only discover this when you become so desperate and horny that you call over a few ‘friends’ to help you out. By the time they leave you’re practically pinned down by the size of your belly, obviously with a much bigger clutch than the last one considering that you're already bigger than you were when you gave birth the first time. A few more visits like that and a trip to a kink club later, you're begging your friend to fuck you through your labor, your body torn between pain and pleasure as contractions take hold of you.
The third time is even harder to figure out. You have a few growth spurts, but nothing crazy. In fact, your belly is still small enough to be hideable, and that disappoints you more than you thought it would. Then one day you find yourself invited to a friend's barbecue, a family friendly event so you wear a big baggy shirt. Your friends all know the situation, but you don't really feel like explaining to kids or anyone not in the know your situation. Which works out fine, at least until one of your friends' babies starts to cry. Fully wailing, as their parents bring them to a quieter, more private spot to soothe their child, but the damage has already been done. To you at least, who had suddenly grown to looking full term with a singleton. Thankfully, your friends run interference on explaining the situation to others and make sure you're nice and comfortable the rest of your time there. One of your friends even gives you a ride home, and is more than happy to, um, “watch a movie and chill” with you for the rest of the evening. To test your new theory, you put on a movie that you vaguely remember had a scene or so with a crying baby. To both of your pleasure, your theory is proven right, your friend fondling your growing belly as they fuck you over the back of your couch, telling you that someday they'll put their own babies in your womb.
This time, it also seems to have an effect on your chest as well. Before, your chest had swollen with milk, obviously your body didn't fully get the memo you weren't carrying human offspring, so you'd found yourself with a pretty cute pair of tits. But with your third clutch, it seems like every time your belly grows, your breasts grow too. There may be some truth to what you'd heard about the sound of a baby's cry making a human suddenly start lactating, because it seems like your shirt is always soaked through after a growth spurt. It gets to the point where you buy a pump, and begin milking yourself like a cow, ear buds in your ears as you listen to a soundtrack of babies crying, and settle into a comfy spot to prepare for another massive growth session.
Each time it's a different thing, some harder to figure out than others. Sometimes you don't figure it out at all, like when your pregnancy only progressed when your blood sugar hit a certain level or when someone with brown eyes looked a little too long at your belly. One time, it was every time you sneezed. That one was only difficult to figure out because you started growing during allergy season, and so many other things were happening that it was hard to pinpoint what it was that was causing you to constantly grow. Then allergy season ended, and you had to waddle around, your belly low and heavy, for months, not realizing you were one good sneeze away from squatting down and pushing out your clutch of slime eggs. Another time, it was when you smelled werewolf pheromones, which isn't something the human nose can pick up, so it took you a little while to figure out. It turns out, a friend of yours was secretly a werewolf, and once you figured it out they were more than happy to introduce you to a few more ‘friends’ who were especially excited to help you out.
You don't even realize when the witch's curse ends (she's up to her ears in slime eggs at that point). You're simply confused that instead of your belly growing in sudden explosive spurts, your belly has been steadily growing at a slow pace. And you're not quite as big as you're used to. And the slight shifting of the slime eggs seems stronger than it has previously. And by the time it comes to give birth, the eggs feel less squishy than they had previously as the first one begins to move through your birth canal. Imagine your shock as the ‘friend’ you called over to fuck you through yet another birth tells you that its not a slime egg crowning between your legs, but a full head of hair. Turns out they made good on the promise they made all that time ago, that someday their own babies would be growing in your belly. Just like they promised they would when you called them over, both of you still convinced you were carrying a clutch as usual, they make sure to overwhelm you with pleasure for the rest of your birth, until you have two human babies suckling at your teats. You make them promise to fill you with their babies as soon as possible, as you wonder to yourself if that witch would be willing to make you a fertility potion or two so you can get as big and round as possible.