Research suggests that Fundal Height is unreliable, though it is a useful means of -detecting- the presence of multiples. This "rule of thumb" came about before ultrasound was a thing, so I doubt there's been much further study beyond that.
Also, as the calculator can't estimate weight gain from fat, it's difficult to say what the measurements end up becoming.
But for the sake of speculating, pregnant women do put on a few protective pounds…
It's generally agreed that pregnant women also go "up a cup" or two on average, but I'm not 100% on how that affects band size and/or bust measure. The butt size would increase somewhat proportionately with it though.
The waist measure was a particular problem we considered too. "How much percentage of her belly circumference is added to the waist measure, and what percent of the waist measure is overlapped by Tess's growing belly?"
Taken to infinity, the arc of the belly sphere cuts directly through the middle of her waist; a distance of 13.66", with the rest of her waist being 15". (1.66" is thus an upper bound)
Realistically though, a full term belly only cuts through maybe a foot of the full 30" around, leaving 18" normal, so you would add 6" to the belly circumference to get the measure.
There would have to be some sort of waist measure-influence function, tapering off as her belly circumference dominates the overall numbers. Think you could make that work?