If that's not good enough, think of it like a tree falling in the woods with no one to hear. If a manga character is at full term with no one to draw them, does the kid come out?
Since the girls seem to have gone through two trimesters of pregnancy over the course of one page and "a few seconds", we should hope the growth rate does not remain consistent. They'd have to go into labor and give birth between pages or end up five months overdue by the next panel. We must assume a limit on either their growth or elasticity, because the manga's later chapters do not feature a trio of schoolgirl bellies blotting out the sun.
It's possible the very act of being observed through their depiction in the manga–the "Gag"–is what advances their condition, bringing them from the quantumly uncertain realm of "possibly pregnant" into the concrete "definitely pregnant". Without being acted upon by an outside force (in this case, the mangaka drawing them), we might assume that as an object at rest stays at rest, a pregnant stomach continues to peek out from under that uniform. If offpanel pregnancies continue at a non-accelerated pace, the girls would have a few months at most to be featured in a new Gag or be forcibly introduced offpanel to the wild world of single motherhood. (Plus or minus the child support they could collectively wring out of one ikemen antagonist.) If instead their biological clocks are manually wound by the artist's pencil, the undrawn girls could spend the rest of their high school, college, work, and love lives in the third trimester.
We can only speculate as to the nature of a hypothetical follow-up or "Second Gag". If any of the girls were caught waddling in the background of a crowd scene, it would make their pregnancy canon. Those seeking to retcon their transformation away would find it that much more difficult to undo. On the other hand, any girl the author uses to deliver a throwaway line to the protagonist with a trim belly might never have to deliver anything else–it could render the entire experience as non-canon as a fever dream or make it so that everything returned "back to normal" after their prettyboy paramour was defeated.
If we hold this to be true, we can separate comic-book Japan into "high-risk" and "low-risk" scenes. The optimal low-risk place to appear would be any form of group fanservice or "cheesecake" chapter, where even forgotten female extras get dredged up with new supermodel figures to bait fans. A "beach episode" would be ideal: no matter how round or overdue a girl is, if they can hold it in until the "Umi da!", they're likely to find themselves with beach bodies.
The effectiveness of this principle is demonstrated by the bunnygirl priestess Iyo from Big Order. At the end of one episode the virgin miko instantly finds herself nine months along with the unwanted child of the protagonist, whom she has just met. She spends the entirety of the next episode painfully giving birth, but with the last of her strength manages to hold on through the end credits. As the next episode's opening scene is set in an onsen (Japanese hot spring), she finds her naked body inexplicably restored to perfect fitness with no child to take care of.
It is important to note low-risk is not the same as "no-risk". While it may seem advantageous for a supernaturally pregnant "mob" character to wear the skimpiest outfit possible and strut past every important character in the manga until a crowd shot catches her, this has the risk of becoming "funny". While most mangaka would enjoy drawing a tableau of 8 beautiful women in bikinis, they would also enjoy drawing 7 beautiful women and one conventionally unattractive gag character in bikinis. A poorly staged debut might not only leave a girl with a canon pregnancy, but a comically massive canon pregnancy. Also note pregnant fanservice is rare but not unheard of, so a pregnant character should learn as much as possible about her author's proclivities beforehand before deciding to expose themselves around the protagonist.
On the other hand, a high-risk area would be a motherly scene like a park, day-care, or maternal swim class. The girls should avoid pregnancy jokes as much as possible in case one turns into an on-panel gag, cementing their status as Really Pregnant. Considering the initial speed of the gestation, it also wouldn't be thematically inappropriate to have an instant birth. The girls should never walk too close to an empty baby carriage, lest they double over and fill it up. However, depending on how each individual girl feels about escaping the rigors of childbirth or her pregnancy of indefinite length, anything that might spur a rapid delivery Gag may be considered positive in its own right.
The most dangerously high-risk zones are birth scenes such as hospitals, ambulances, and elevators. These might seem desirable areas to bring the pregnancy to a canon and permanent end, but one risks the worst case scenario: what if the mangaka bothers to draw the girl in labor, but not post-delivery? These extras are already trapped on the quantum boundary between Pregnant and Not Pregnant, and what is birth but just that? Until the author collapses the waveform by drawing the swaddled babe, these girls would be left pushing and pushing an angry arrogant handsome villain baby through their birth canal for eternity.
Knowing all this, it's not surprising the girls never show up again. They probably cut their losses and skipped the manga entirely. Whether they're off raising the next generation of Schwarzschildren or still limiting those baby villains to however hard they can kick a bladder, things could have been far worse.
I hope this answers your question.