Yes they are measured in cm. Normally they divide the womb into 4 quadrants, measure one and multiply by 4 to get the total volume of fluid. Fluid peaks around 28wksish - 32 wks and is steady from there until full-term (37- 40 wks). when it starts to decline. Largest amt of lung development is in the mid-20s.
The big question will be why she has reduced fluid. PROM is one reason (premature rupture of membranes), but could also be a defect in the baby (most oftn renal or urinary as fluid is made from baby's urine or possibly other).
If they need/want a genetic test, it will probably have to be a CVS as they might not have enough fluid for an amino.
Sounds as if the lack of fluid hasn't impacted on the development. With my son he had so little fluid that it lead to physical deformaties (people are designed to be formed in water).
Most often they recommend bedrest. Experimental is to inject sterile saline into womb. Some studeis show if the mother drinks 2L of water this can boost fluid levels.
She should be closely monitored for both fluid levels and baby growth. They will probably try to keep baby in for as long as poss, but babe may need to be born early.
Hope she gets to a dr soon -- and new hospital.
My thoughts are with you and your family.