It can get obnoxious, especially when it's used over and over again. Typically, if I use it it's initially for a pseudo-baby bump and then at "full term". Then I base the size off that (double, triple, etc.) and go off into diameter in feet. The funny thing is, so many people have used the pregnancy "x weeks" analogy that I have partially memorized the sort of sizes a woman's womb is at during varying points of time, just from the amount of times I've looked up pregnancy progression videos and the like. I don't like that system very much to be honest, because they're really nothing more than coordinates. There's nothing real in those numbers, despite their relation to pregnancy. Like you said, hardly anyone can know at the top of their head the size of a womb during the 23rd week of a triplet pregnancy. Fruit and sports equipment work so much better, especially in my case. I've touched a soccer ball before, but the same cannot be said of a woman's pregnant belly.
That ugly truth about inflation is precisely why I dislike eroticism taken to an extreme/hyperbole in the fiction. It takes a suspension of disbelief that I find hard to achieve unless in the right mood. I prefer good storytelling, because often times the overt erotica sacrificed for better storytelling is made up for in many ways (good characters, dialogue, description/events) that can make the story just as sexy. Not erotic per se, but definitely sexy.
That's exactly how I'd picture it going. She knows both the events, the result, and the aftermath. And none of them are good. They say that ignorance is bliss, and nothing could be more true for that scenario. Better to hope that the ambulance has nitrous than accept your fate. Of all the emotions that characters express right at that moment before the bang, I have to say that I like sheer terror and stoic acceptance the most. Like in the story Popperchicks, one of the main characters states matter-of-factly "it's me" and then explodes. (Could aforementioned sadism be a factor?) That sort of feeling of just throwing in the towel and accepting the inevitable also seems very real. Many people have many different ways of reacting to things. To think that not a single person would have an 'it doesn't matter, I'm doomed' epiphany toward the end isn't right. Surely it would be rare, but not impossible.